Organometallic complexes as phosphorescent emitters in organic LEDs

ABSTRACT

Organic light emitting devices are described wherein the emissive layer comprises a host material containing an emissive molecule, which molecule is adapted to luminesce when a voltage is applied across the heterostructure, and the emissive molecule is selected from the group of phosphorescent organometallic complexes, including cyclometallated platinum, iridium and osmium complexes. The organic light emitting devices optionally contain an exciton blocking layer. Furthermore, improved electroluminescent efficiency in organic light emitting devices is obtained with an emitter layer comprising organometallic complexes of transition metals of formula L 2 MX, wherein L and X are distinct bidentate ligands. Compounds of this formula can be synthesized more facilely than in previous approaches and synthetic options allow insertion of fluorescent molecules into a phosphorescent complex, ligands to fine tune the color of emission, and ligands to trap carriers.

[0001] This is a continuation-in-part of application Ser. No.09/274,609, filed Mar. 23, 1999; application Ser. No. 09/452,346, filedDec. 1, 1999; and application Ser. No. 09/311,126, filed May 13, 1999,which is a continuation-in-part of application Ser. No. 09/153,144,filed Sep. 14, 1998, now U.S. Pat. No. 6,097,147.

FIELD OF INVENTION

[0002] The present invention is directed to organic light emittingdevices (OLEDs) comprised of emissive layers that contain anorganometallic phosphorescent compound.

BACKGROUND OF THE INVENTION

[0003] Organic light emitting devices (OLEDs) are comprised of severalorganic layers in which one of the layers is comprised of an organicmaterial that can be made to electroluminesce by applying a voltageacross the device, C. W. Tang et al., Appl. Phys. Lett. 1987, 51, 913.Certain OLEDs have been shown to have sufficient brightness, range ofcolor and operating lifetimes for use as a practical alternativetechnology to LCD-based full color flat-panel displays (S. R. Forrest,P. E. Burrows and M. E. Thompson, Laser Focus World, February 1995).Since many of the thin organic films used in such devices aretransparent in the visible spectral region, they allow for therealization of a completely new type of display pixel in which red (R),green (G), and blue (B) emitting OLEDs are placed in a verticallystacked geometry to provide a simple fabrication process, a small R-G-Bpixel size, and a large fill factor, International Patent ApplicationNo. PCT/US95/15790.

[0004] A transparent OLED (TOLED), which represents a significant steptoward realizing high resolution, independently addressable stackedR-G-B pixels, was reported in International Patent Application No.PCT/US97/02681 in which the TOLED had greater than 71% transparency whenturned off and emitted light from both top and bottom device surfaceswith high efficiency (approaching 1% quantum efficiency) when the devicewas turned on. The TOLED used transparent indium tin oxide (ITO) as thehole-injecting electrode and a Mg—Ag-ITO electrode layer forelectron-injection. A device was disclosed in which the ITO side of theMg—Ag-ITO electrode layer was used as a hole-injecting contact for asecond, different color-emitting OLED stacked on top of the TOLED. Eachlayer in the stacked OLED (SOLED) was independently addressable andemitted its own characteristic color. This colored emission could betransmitted through the adjacently stacked, transparent, independentlyaddressable, organic layer or layers, the transparent contacts and theglass substrate, thus allowing the device to emit any color that couldbe produced by varying the relative output of the red and bluecolor-emitting layers.

[0005] The PCT/US95/15790 application disclosed an integrated SOLED forwhich both intensity and color could be independently varied andcontrolled with external power supplies in a color tunable displaydevice. The PCT/US95/15790 application, thus, illustrates a principlefor achieving integrated, full color pixels that provide high imageresolution, which is made possible by the compact pixel size.Furthermore, relatively low cost fabrication techniques, as comparedwith prior art methods, may be utilized for making such devices.

[0006] Because light is generated in organic materials from the decay ofmolecular excited states or excitons, understanding their properties andinteractions is crucial to the design of efficient light emittingdevices currently of significant interest due to their potential uses indisplays, lasers, and other illumination applications. For example, ifthe symmetry of an exciton is different from that of the ground state,then the radiative relaxation of the exciton is disallowed andluminescence will be slow and inefficient. Because the ground state isusually anti-symmetric under exchange of spins of electrons comprisingthe exciton, the decay of a symmetric exciton breaks symmetry. Suchexcitons are known as triplets, the term reflecting the degeneracy ofthe state. For every three triplet excitons that are formed byelectrical excitation in an OLED, only one symmetric state (or singlet)exciton is created. (M. A. Baldo, D. F. O'Brien, M. E. Thompson and S.R. Forrest, Very high-efficiency green organic light-emitting devicesbased on electrophosphorescence, Applied Physics Letters, 1999, 75,4-6.) Luminescence from a symmetry-disallowed process is known asphosphorescence. Characteristically, phosphorescence may persist for upto several seconds after excitation due to the low probability of thetransition. In contrast, fluorescence originates in the rapid decay of asinglet exciton. Since this process occurs between states of likesymmetry, it may be very efficient.

[0007] Many organic materials exhibit fluorescence from singletexcitons. However, only a very few have been identified which are alsocapable of efficient room temperature phosphorescence from triplets.Thus, in most fluorescent dyes, the energy contained in the tripletstates is wasted. However, if the triplet excited state is perturbed,for example, through spin-orbit coupling (typically introduced by thepresence of a heavy metal atom), then efficient phosphoresence is morelikely. In this case, the triplet exciton assumes some singlet characterand it has a higher probability of radiative decay to the ground state.Indeed, phosphorescent dyes with these properties have demonstrated highefficiency electroluminescence.

[0008] Only a few organic materials have been identified which showefficient room temperature phosphorescence from triplets. In contrast,many fluorescent dyes are known (C. H. Chen, J. Shi, and C. W. Tang,“Recent developments in molecular organic electroluminescent materials,”Macromolecular Symposia, 1997, 125, 1-48; U. Brackmann, LambdachromeLaser Dyes (Lambda Physik, Gottingen, 1997)) and fluorescentefficiencies in solution approaching 100% are not uncommon. (C. H. Chen,1997, op. cit.) Fluorescence is also not affected by triplet-tripletannihilation, which degrades phosphorescent emission at high excitationdensities. (M. A. Baldo, et al., “High efficiency phosphorescentemission from organic electroluminescent devices,” Nature, 1998, 395,151-154; M. A. Baldo, M. E. Thompson, and S. R. Forrest, “An analyticmodel of triplet-triplet annihilation in electrophosphorescent devices,”1999). Consequently, fluorescent materials are suited to manyelectroluminescent applications, particularly passive matrix displays.

[0009] To understand the different embodiments of this invention, it isuseful to discuss the underlying mechanistic theory of energy transfer.There are two mechanisms commonly discussed for the transfer of energyto an acceptor molecule. In the first mechanism of Dexter transport (D.L. Dexter, “A theory of sensitized luminescence in solids,” J. Chem.Phys., 1953, 21, 836-850), the exciton may hop directly from onemolecule to the next. This is a short-range process dependent on theoverlap of molecular orbitals of neighboring molecules. It alsopreserves the symmetry of the donor and acceptor pair (E. Wigner and E.W. Wittmer, Uber die Struktur der zweiatomigen Molekelspektren nach derQuantenmechanik, Zeitschrift fur Physik, 1928, 51, 859-886; M.Klessinger and J. Michl, Excited states and photochemistry of organicmolecules (VCH Publishers, New York, 1995)). Thus, the energy transferof Eq. (1) is not possible via Dexter mechanism. In the second mechanismof Förster transfer (T. Förster, Zwischenmolekulare Energiewanderung andFluoreszenz, Annalen der Physik, 1948, 2, 55-75; T. Forster, Fluoreszenzorganisciler Verbindugen (Vandenhoek and Ruprecht, Gottinghen, 1951),the energy transfer of Eq. (1) is possible. In Forster transfer, similarto a transmitter and an antenna, dipoles on the donor and acceptormolecules couple and energy may be transferred. Dipoles are generatedfrom allowed transitions in both donor and acceptor molecules. Thistypically restricts the Forster mechanism to transfers between singletstates.

[0010] Nevertheless, as long as the phosphor can emit light due to someperturbation of the state such as due to spin-orbit coupling introducedby a heavy metal atom, it may participate as the donor in Forstertransfer. The efficiency of the process is determined by the luminescentefficiency of the phosphor (F Wilkinson, in Advances in Photochemistry(eds. W. A. Noyes, G. Hammond, and J. N. Pitts), pp. 241-268, John Wiley& Sons, New York, 1964), i.e., if a radiative transition is moreprobable than a non-radiative decay, then energy transfer will beefficient. Such triplet-singlet transfers were predicted by Forster (T.Forster, “Transfer mechanisms of electronic excitation,” Discussions ofthe Faraday Society, 1959, 27, 7-17) and confirmed by Ermolaev andSveshnikova (V. L. Ermolaev and E. B. Sveshnikova, “Inductive-resonancetransfer of energy from aromatic molecules in the triplet state,”Doklady Akademii Nauk SSSR, 1963, 149, 1295-1298), who detected theenergy transfer using a range of phosphorescent donors and fluorescentacceptors in rigid media at 77K or 90K. Large transfer distances areobserved; for example, with triphenylamine as the donor and chrysoidineas the acceptor, the interaction range is 52 Å.

[0011] The remaining condition for Forster transfer is that theabsorption spectrum should overlap the emission spectrum of the donorassuming the energy levels between the excited and ground statemolecular pair are in resonance. In one example of this application, weuse the green phosphor fac tris(2-phenylpyridine)iridium (Ir(Ppy)₃; M.A. Baldo, et al., Appl. Phys. Lett., 1999, 75, 4-6) and the redfluorescent dye[2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-ylidene]propane-dinitrile] (DCM2“; C. W. Tang, S. A.VanSlyke, and C. H. Chen, “Electroluminescence of doped organic films,”J. Appl. Phys., 1989, 65, 3610-3616). DCM2 absorbs in the green, and,depending on the local polarization field (V. Bulovic, et al., “Bright,saturated, red-to-yellow organic light-emitting devices based onpolarization-induced spectral shifts,” Chem. Phys. Lett., 1998, 287,455-460), it emits at wavelengths between λ=570 nm and λ=650 nm.

[0012] It is possible to implement Forster energy transfer from atriplet state by doping a fluorescent guest into a phosphorescent hostmaterial. Unfortunately, such systems are affected by competitive energytransfer mechanisms that degrade the overall efficiency. In particular,the close proximity of the host and guest increase the likelihood ofDexter transfer between the host to the guest triplets. Once excitonsreach the guest triplet state, they are effectively lost since thesefluorescent dyes typically exhibit extremely inefficientphosphorescence.

[0013] To maximize the transfer of host triplets to fluorescent dyesinglets, it is desirable to maximize Dexter transfer into the tripletstate of the phosphor while also minimizing transfer into the tripletstate of the fluorescent dye. Since the Dexter mechanism transfersenergy between neighboring molecules, reducing the concentration of thefluorescent dye decreases the probability of triplet-triplet transfer tothe dye. On the other hand, long range Forster transfer to the singletstate is unaffected. In contrast, transfer into the triplet state of thephosphor is necessary to harness host triplets, and may be improved byincreasing the concentration of the phosphor.

[0014] Devices whose structure is based upon the use of layers oforganic optoelectronic materials generally rely on a common mechanismleading to optical emission. Typically, this mechanism is based upon theradiative recombination of a trapped charge. Specifically, OLEDs arecomprised of at least two thin organic layers separating the anode andcathode of the device. The material of one of these layers isspecifically chosen based on the material's ability to transport holes,a “hole transporting layer” (HTL), and the material of the other layeris specifically selected according to its ability to transportelectrons, an “electron transporting layer” (ETL). With such aconstruction, the device can be viewed as a diode with a forward biaswhen the potential applied to the anode is higher than the potentialapplied to the cathode. Under these bias conditions, the anode injectsholes (positive charge carriers) into the hole transporting layer, whilethe cathode injects electrons into the electron transporting layer. Theportion of the luminescent medium adjacent to the anode thus forms ahole injecting and transporting zone while the portion of theluminescent medium adjacent to the cathode forms an electron injectingand transporting zone. The injected holes and electrons each migratetoward the oppositely charged electrode. When an electron and holelocalize on the same molecule, a Frenkel exciton is formed.Recombination of this short-lived state may be visualized as an electrondropping from its conduction potential to a valence band, withrelaxation occurring, under certain conditions, preferentially via aphotoemissive mechanism. Under this view of the mechanism of operationof typical thin-layer organic devices, the electroluminescent layercomprises a luminescence zone receiving mobile charge carriers(electrons and holes) from each electrode.

[0015] As noted above, light emission from OLEDs is typically viafluorescence or phosphorescence. There are issues with the use ofphosphorescence. It has been noted that phosphorescent efficiencydecreases rapidly at high current densities. It may be that longphosphorescent lifetimes cause saturation of emissive sites, andtriplet-triplet annihilation may produce efficiency losses. Anotherdifference between fluorescence and phosphorescence is that energytransfer of triplets from a conductive host to a luminescent guestmolecule is typically slower than that of singlets; the long rangedipole-dipole coupling (Förster transfer) which dominates energytransfer of singlets is (theoretically) forbidden for triplets by theprinciple of spin symmetry conservation. Thus, for triplets, energytransfer typically occurs by diffusion of excitons to neighboringmolecules (Dexter transfer); significant overlap of donor and acceptorexcitonic wavefunctions is critical to energy transfer. Another issue isthat triplet diffusion lengths are typically long (e.g., >1400 Å)compared with typical singlet diffusion lengths of about 200 Å. Thus, ifphosphorescent devices are to achieve their potential, device structuresneed to be optimized for triplet properties. In this invention, weexploit the property of long triplet diffusion lengths to improveexternal quantum efficiency.

[0016] Successful utilization of phosphorescence holds enormous promisefor organic electroluminescent devices. For example, an advantage ofphosphorescence is that all excitons (formed by the recombination ofholes and electrons in an EL), which are (in part) triplet-based inphosphorescent devices, may participate in energy transfer andluminescence in certain electroluminescent materials. In contrast, onlya small percentage of excitons in fluorescent devices, which aresinglet-based, result in fluorescent luminescence.

[0017] An alternative is to use phosphorescence processes to improve theefficiency of fluorescence processes. Fluorescence is in principle 75%less efficient due to the three times higher number of symmetric excitedstates.

[0018] Because one typically has at least one electron transportinglayer and at least one hole transporting layer, one has layers ofdifferent materials, forming a heterostructure. The materials thatproduce the electroluminescent emission are frequently the samematerials that function either as the electron transporting layer or asthe hole transporting layer. Such devices in which the electrontransporting layer or the hole transporting layer also functions as theemissive layer are referred to as having a single heterostructure.Alternatively, the electroluminescent material may be present in aseparate emissive layer between the hole transporting layer and theelectron transporting layer in what is referred to as a doubleheterostructure. The separate emissive layer may contain the emissivemolecule doped into a host or the emissive layer may consist essentiallyof the emissive molecule.

[0019] That is, in addition to emissive materials that are present asthe predominant component in the charge carrier layer, that is, eitherin the hole transporting layer or in the electron transporting layer,and that function both as the charge carrier material as well as theemissive material, the emissive material may be present in relativelylow concentrations as a dopant in the charge carrier layer. Whenever adopant is present, the predominant material in the charge carrier layermay be referred to as a host compound or as a receiving compound.Materials that are present as host and dopant are selected so as to havea high level of energy transfer from the host to the dopant material. Inaddition, these materials need to be capable of producing acceptableelectrical properties for the OLED. Furthermore, such host and dopantmaterials are preferably capable of being incorporated into the OLEDusing starting materials that can be readily incorporated into the OLEDby using convenient fabrication techniques, in particular, by usingvacuum-deposition techniques.

[0020] The exciton blocking layer used in the devices of the presentinvention (and previously disclosed in U.S. application Ser. No.09/154,044) substantially blocks the diffusion of excitons, thussubstantially keeping the excitons within the emission layer to enhancedevice efficiency. The material of blocking layer of the presentinvention is characterized by an energy difference (“band gap”) betweenits lowest unoccupied molecular orbital (LUMO) and its highest occupiedmolecular orbital (HOMO). in accordance with the present invention, thisband gap substantially prevents the diffusion of excitons through theblocking layer, yet has only a minimal effect on the turn-on voltage ofa completed electroluminescent device. The band gap is thus preferablygreater than the energy level of excitons produced in an emission layer,such that such excitons are not able to exist in the blocking layer.Specifically, the band gap of the blocking layer is at least as great asthe difference in energy between the triplet state and the ground stateof the host.

[0021] It is desirable for OLEDs to be fabricated using materials thatprovide electroluminescent emission in a relatively narrow band centerednear selected spectral regions, which correspond to one of the threeprimary colors, red, green and blue so that they may be used as acolored layer in an OLED or SOLED. It is also desirable that suchcompounds be capable of being readily deposited as a thin layer usingvacuum deposition techniques so that they may be readily incorporatedinto an OLED that is prepared entirely from vacuum-deposited organicmaterials.

[0022] Co-pending application U.S. Ser. No. 08/774,087, filed Dec. 23,1996, now U.S. Pat. No. 6,048,630, is directed to OLEDs containingemitting compounds that produce a saturated red emission.

SUMMARY OF THE INVENTION

[0023] The present invention is directed to organic light emittingdevices wherein the emissive layer comprises an emissive molecule,optionally with a host material (wherein the emissive molecule ispresent as a dopant in said host material), which molecule is adapted toluminesce when a voltage is applied across the heterostructure, whereinthe emissive molecule is selected from the group of phosphorescentorganometallic complexes. The emissive molecule may be further selectedfrom the group of phosphorescent organometallic platinum, iridium orosmium complexes and may be still further selected from the group ofphosphorescent cyclometallated platinum, iridium or osmium complexes. Aspecific example of the emissive molecule is factris(2-phenylpyridine)iridium, denoted (Ir(ppy)₃) of formula

[0024] [In this, and later figures herein, we depict the dative bondfrom nitrogen to metal (here, Ir) as a straight line.]

[0025] The general arrangement of the layers is hole transporting layer,emissive layer, and electron transporting layer. For a hole conductingemissive layer, one may have an exciton blocking layer between theemissive layer and the electron transporting layer. For an electronconducting emissive layer, one may have an exciton blocking layerbetween the emissive layer and the hole transporting layer. The emissivelayer may be equal to-the-hole transporting layer (in which case theexciton blocking layer is near or at the anode) or to the electrontransporting layer (in which case the exciton blocking layer is near orat the cathode).

[0026] The emissive layer may be formed with a host material in whichthe emissive molecule resides as a guest or the emissive layer may beformed of the emissive molecule itself. In the former case, the hostmaterial may be a hole-transporting material selected from the group ofsubstituted tri-aryl amines. The host material may be anelectron-transporting material selected from the group of metalquinoxolates, oxadiazoles and triazoles. An example of a host materialis 4,4′-N,N′-dicarbazole-biphenyl (CBP), which has the formula:

[0027] The emissive layer may also contain a polarization molecule,present as a dopant in said host material and having a dipole moment,that affects the wavelength of light emitted when said emissive dopantmolecule luminesces.

[0028] A layer formed of an electron transporting material is used totransport electrons into the emissive layer comprising the emissivemolecule and the (optional) host material. The electron transportingmaterial may be an electron-transporting matrix selected from the groupof metal quinoxolates, oxadiazoles and triazoles. An example of anelectron transporting material is tris-(8-hydroxyquinoline)aluminum(Alq₃).

[0029] A layer formed of a hole transporting material is used totransport holes into the emissive layer comprising the emissive moleculeand the (optional) host material. An example of a hole transportingmaterial is 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl [“c-NPD”].

[0030] The use of an exciton blocking layer (“barrier layer”) to confineexcitons within the luminescent layer (“luminescent zone”) is greatlypreferred. For a hole-transporting host, the blocking layer may beplaced between the luminescent layer and the electron transport layer.An example of a material for such a barrier layer is2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (also called bathocuproineor BCP), which has the formula:

[0031] For a situation with a blocking layer between a hole-conductinghost and the electron transporting layer (as is the case in Example 2below), one seeks the following characteristics, which are listed inorder of relative importance.

[0032] 1. The difference in energy between the LUMO and HOMO of theblocking layer is greater than the difference in energy between thetriplet and ground state singlet of the host material.

[0033] 2. Triplets in the host material are not quenched by the blockinglayer.

[0034] 3. The ionization potential (IP) of the blocking layer is greaterthan the ionization potential of the host. (Meaning that holes are heldin the host.)

[0035] 4. The energy level of the LUMO of the blocking layer and theenergy level of the LUMO of the host are sufficiently close in energysuch that there is less than 50% change in the overall conductivity ofthe device.

[0036] 5. The blocking layer is as thin as possible subject to having athickness of the layer that is sufficient to effectively block thetransport of excitons from the emissive layer into the adjacent layer.

[0037] That is, to block excitons and holes, the ionization potential ofthe blocking layer should be greater than that of the HTL, while theelectron affinity of the blocking layer should be approximately equal tothat of the ETL to allow for facile transport of electrons.

[0038] [For a situation in which the emissive (“emitting”) molecule isused without a hole transporting host, the above rules for selection ofthe blocking layer are modified by replacement of the word “host” by“emitting molecule.”]

[0039] For the complementary situation with a blocking layer between aelectron-conducting host and the hole-transporting layer one seekscharacteristics (listed in order of importance):

[0040] 1. The difference in energy between the LUMO and HOMO of theblocking layer is greater than the difference in energy between thetriplet and ground state singlet of the host material.

[0041] 2. Triplets in the host material are not quenched by the blockinglayer.

[0042] 3. The energy of the LUMO of the blocking layer is greater thanthe energy of the LUMO of the (electron-transporting) host. (Meaningthat electrons are held in the host.)

[0043] 4. The ionization potential of the blocking layer and theionization potential of the host are such that holes are readilyinjected from the blocker into the host and there is less than a 50%change in the overall conductivity of the device.

[0044] 5. The blocking layer is as thin as possible subject to having athickness of the layer that is sufficient to effectively block thetransport of excitons from the emissive layer into the adjacent layer.

[0045] [For a situation in which the emissive (“emitting”) molecule isused without an electron transporting host, the above rules forselection of the blocking layer are modified by replacement of the word“host” by “emitting molecule.”]

[0046] The present invention covers articles of manufacture comprisingOLEDs comprising a new family of phosphorescent materials, which can beused as dopants in OLEDs, and methods of manufacturing the articles.These phosphorescent materials are cyclometallated platinum, iridium orosmium complexes, which provide electroluminiscent emission at awavelength between 400 nm and 700 nm. The present invention is furtherdirected to OLEDs that are capable of producing an emission that willappear blue, that will appear green, and that will appear red.

[0047] More specifically, OLEDs of the present invention comprise, forexample, an emissive layer comprised of platinum (II) complexed withBis[2-(2-phenyl)pyridinato-N,C2], Bis[2-(2′-thienyl)pyridinato-N,C3],and Bis[benzo(h)quinolinato-N,C]. The compoundcis-Bis[2-(2′-thienyl)pyridinato-N,C3] Pt(II) gives a strong orange toyellow emission.

[0048] The invention is further directed to emissive layers wherein theemissive molecule is selected from the group of phosphorescentorganometallic complexes, wherein the emissive molecule containssubstituents selected from the class of electron donors and electronacceptors. The emissive molecule may be further selected from the groupof phosphorescent organometallic platinum, iridium or osmium complexesand may be still further selected from the group of phosphorescentcyclometallated platinum, iridium or osmium complexes, wherein theorganic molecule contains substituents selected from the class ofelectron donors and electron acceptors.

[0049] The invention is further directed to an organic light emittingdevice comprising a heterostructure for producing luminescence, whereinthe emissive layer comprises a host material, an emissive molecule,present as a dopant in said host material, adapted to luminesce when avoltage is applied across the heterostructure, wherein the emissivemolecule is selected from the group consisting of cyclometallatedplatinum, iridium or osmium complexes and wherein there is apolarization molecule, present as a dopant in the host material, whichpolarization molecule has a dipole moment and which polarizationmolecule alters the wavelength of the luminescent light emitted by theemissive dopant molecule. The polarization molecule may be an aromaticmolecule substituted by electron donors and electron acceptors.

[0050] The present invention is directed to OLEDs, and a method offabricating OLEDs, in which emission from the device is obtained via aphosphorescent decay process wherein the phosphorescent decay rate israpid enough to meet the requirements of a display device. Morespecifically, the present invention is directed to OLEDs comprised of amaterial that is capable of receiving the energy from an exciton singletor triplet state and emitting that energy as phosphorescent radiation.

[0051] The OLEDs of the present invention may be used in substantiallyany type of device which is comprised of an OLED, for example, in OLEDsthat are incorporated into a larger display, a vehicle, a computer, atelevision, a printer, a large area wall, theater or stadium screen, abillboard or a sign.

[0052] The present invention is also directed to complexes of formula LL′ L″ M, wherein L, L′, and L″ are distinct bidentate ligands and M is ametal of atomic number greater than 40 which forms an octahedral complexwith the three bidentate ligands and is preferably a member of the thirdrow (of the transition series of the periodic table) transition metals,most preferably Ir and Pt. Alternatively, M can be a member of thesecond row transition metals, or of the main group metals, such as Zrand Sb. Some of such organometallic complexes electroluminesce, withemission coming from the lowest energy ligand or MLCT state. Suchelectroluminescent compounds can be used in the emitter layer of organiclight emitting diodes, for example, as dopants in a host layer of anemitter layer in organic light emitting diodes. This invention isfurther directed to organometallic complexes of formula L L′ L″ M,wherein L, L′, and L″ are the same (represented by L₃M) or different(represented by L L′ L″ M), wherein L, L′, and L″ are bidentate,monoanionic ligands, wherein M is a metal which forms octahedralcomplexes, is preferably a member of the third row of transition metals,more preferably Ir or Pt, and wherein the coordinating atoms of theligands comprise sp² hybridized carbon and a heteroatom. The inventionis further directed to compounds of formula L₂MX, wherein L and X aredistinct bidentate ligands, wherein X is a monoanionic bidentate ligand,wherein L coordinates to M via atoms of L comprising sp² hybridizedcarbon and heteroatoms, and wherein M is a metal forming an octahedralcomplex, preferably iridium or platinum. It is generally expected thatthe ligand L participates more in the emission process than does X. Theinvention is directed to meridianal isomers of L₃M wherein theheteroatoms (such as nitrogen) of two ligands L are in a transconfiguration. In the embodiment in which M is coordinated with an sp²hybridized carbon and a heteroatom of the ligand, it is preferred thatthe ring comprising the metal M, the sp² hybridized carbon and theheteroatom contains 5 or 6 atoms. These compounds can serve as dopantsin a host layer which functions as a emitter layer in organic lightemitting diodes.

[0053] Furthermore, the present invention is directed to the use ofcomplexes of transition metal species M with bidentate ligands L and Xin compounds of formula L₂MX in the emitter layer of organic lightemitting diodes. A preferred embodiment is compounds of formula L₂IrX,wherein L and X are distinct bidentate ligands, as dopants in a hostlayer functioning as an emitter layer in organic light emitting diodes.

[0054] The present invention is also directed to an improved synthesisof organometallic molecules which function as emitters in light emittingdevices. These compounds of this invention can be made according to thereaction:

L₂M(∥-Cl)₂ML₂+XH→L₂MX+HCl

[0055] wherein L₂M(μ-Cl)₂ML₂ is a chloride bridged dimer with L abidentate ligand, and M a metal such as Ir; XH is a Bronsted acid whichreacts with bridging chloride and serves to introduce a bidentate ligandX, where XH can be, for example, acetylacetone, 2-picolinic acid, orN-methylsalicyclanilide, and H represents hydrogen. The method involvescombining the L₂M(μ-Cl)₂ML₂ chloride bridged dimer with the XH entity.The resultant product of the form L₂MX has approximate octahedraldisposition of the bidentate ligands L, L, and X about M.

[0056] The resultant compounds of formula L₂MX can be used asphosphorescent emitters in organic light emitting devices. For example,the compound wherein L=(2-phenylbenzothiazole), X=acetylacetonate, andM=Ir (the compound abbreviated as BTIr) when used as a dopant in4,4′-N,N′-dicarbazole-biphenyl (CBP) (at a level 12% by mass) to form anemitter layer in an OLED shows a quantum efficiency of 12%. Forreference, the formula for CBP is:

[0057] The synthetic process to make L₂MX compounds of the presentinvention may be used advantageously in a situation in which L, byitself, is fluorescent but the resultant L₂MX is phosphorescent. Onespecific example of this is where L=coumarin-6.

[0058] The synthetic process of the present invention facilitates thecombination of L and X pairs of certain desirable characteristics. Forexample, the present invention is further directed to the appropriateselection of L and X to allow color tuning of the complex L₂MX relativeto L₃M. For example, Ir(ppy)₃ and (ppy)₂Ir(acac) both give strong greenemission with a λ_(max) of 510 nm (ppy denotes phenyl pyridine).However, if the X ligand is formed from picolinic acid instead of fromacetylacetone, there is a small blue shift of about 15 nm.

[0059] Furthermore, the present invention is also directed to aselection of X such that it has a certain HOMO level relative to the L₃Mcomplex so that carriers (holes or electrons) might be trapped on X (oron L) without a deterioration of emission quality. In this way, carriers(holes or electrons) which might otherwise contribute to deleteriousoxidation or reduction of the phosphor would be impeded. The carrierthat is remotely trapped could readily recombine with the oppositecarrier either intramolecularly or with the carrier from an adjacentmolecule.

[0060] The present invention, and its various embodiments, are discussedin more detail in the examples below. However, the embodiments mayoperate by different mechanisms. Without limitation and without limitingthe scope of the invention, we discuss the different mechanisms by whichvarious embodiments of the invention may operate.

BRIEF DESCRIPTION OF THE DRAWINGS

[0061]FIG. 1. Electronic absorbance spectra of Pt(thpy)₂, Pt(thq)₂, andPt(bph)(bpy).

[0062]FIG. 2. Emission spectra of Pt(thpy)₂, Pt(thq)₂, and Pt(bph)(bpy).

[0063]FIG. 3. Energy transfer from polyvinylcarbazole (PVK) to Pt(thpy)₂in the solid film.

[0064]FIG. 4. Characteristics of OLED with Pt(thpy)₂ dopant: (a) I-Vcharacteristic; (b) Light output curve.

[0065]FIG. 5. Quantum efficiency dependence on applied voltage for OLEDwith Pt(thpy)₂ dopant.

[0066]FIG. 6. Characteristics of the OLED device with Pt(thpy)₂ dopant:(a) normalized electroluminescence (EL) spectrum of the device at 22 V(b) CIE diagram based on normalized EL spectrum.

[0067]FIG. 7. Proposed energy level structure of theelectrophosphorescent device of Example 2. The highest occupiedmolecular orbital (HOMO) energy and the lowest unoccupied molecularorbital (LUMO) energy are shown (see I. G. Hill and A. Kahn, J. Appl.Physics (1999)). Note that the HOMO and LUMO levels for Ir(ppy)₃ are notknown. The inset shows structural chemical formulae for: (a) Ir(ppy)₃;(b) CBP; and (c) BCP.

[0068]FIG. 8. The external quantum efficiency of OLEDs using Ir(ppy)₃:CBP luminescent layers. Peak efficiencies are observed for a mass ratioof 6% Ir(ppy)₃ to CBP. The 100% lr(ppy)₃ device has a slightly differentstructure than shown in FIG. 7. In it, the Ir(ppy)₃ layer is 300 A thickand there is no BCP blocking layer. The efficiency of a 6% Ir(ppy)₃: CBPdevice grown without a BCP layer is also shown.

[0069]FIG. 9. The power efficiency and luminance of the 6% Ir(ppy)₃: CBPdevice. At 100 cd/m², the device requires 4.3 V and its power efficiencyis 19 Im/W.

[0070]FIG. 10. The electroluminescent spectrum of 6% Ir(ppy)₃ : CBP.Inset: The Commission Internationale de L'Eclairage (CIE) chromaticitycoordinates of Ir(ppy)₃ in CBP are shown relative to fluorescent greenemitters Alq₃ and poly(p-phenylenevinylene) (PPV).

[0071]FIG. 11. Expected structure of L₂IrX complexes along with thestructure expected for PPIr. Four examples of X ligands used for thesecomplexes are also shown. The structure shown is for an acac derivative.For the other X type ligands, the O—O ligand would be replaced with anN—O ligand.

[0072]FIG. 12. Comparison of facial and meridianal isomers of L₃M.

[0073]FIG. 13. Molecular formulae of mer-isomers disclosed herewith:mer-Ir(ppy)₃ and mer-Ir(bq)₃. PPY (or ppy) denotes phenyl pyridyl and BQ(or bq) denotes 7,8-benzoquinoline.

[0074]FIG. 14. Models of mer-Ir(ppy)₃ (left) and (ppy)₂Ir(acac) (right).

[0075]FIG. 15. (a) Electroluminescent device data (quantum efficiencyvs. current density) for 12% by mass “BTIr” in CBP. BTIr stands for bis(2-phenylbenzothiazole)iridium acetylacetonate; (b) Emission spectrumfrom same device

[0076]FIG. 16. Representative molecule to trap holes (L₂IrX complex).

[0077]FIG. 17. Emission spectrum of Ir(3-MeOppy)₃.

[0078]FIG. 18. Emission spectrum of tpyIrsd.

[0079]FIG. 19. Proton NMR spectrum of tpyIrsd (=typIrsd).

[0080]FIG. 20. Emission spectrum of thpyIrsd.

[0081]FIG. 21. Proton NMR spectrum of thpyIrsd.

[0082]FIG. 22. Emission spectrum of btIrsd.

[0083]FIG. 23. Proton NMR spectrum of btIrsd.

[0084]FIG. 24. Emission spectrum of BQIr.

[0085]FIG. 25. Proton NMR spectrum of BQIr.

[0086]FIG. 26. Emission spectrum of BQIrFA.

[0087]FIG. 27. Emission spectrum of THIr (=thpy; THPIr).

[0088]FIG. 28. Proton NMR spectrum of THPIr.

[0089]FIG. 29. Emission spectra of PPIr.

[0090]FIG. 30. Proton NMR spectrum of PPIr.

[0091]FIG. 31. Emission spectrum of BTHPIr (=BTPIr).

[0092]FIG. 32. Emission spectrum of tpyIr.

[0093]FIG. 33. Crystal structure of tpyIr showing trans arrangement ofnitrogen.

[0094]FIG. 34. Emission spectrum of C6.

[0095]FIG. 35. Emission spectrum of C6Ir.

[0096]FIG. 36. Emission spectrum of PZIrP.

[0097]FIG. 37. Emission spectrum of BONIr.

[0098]FIG. 38. Proton NMR spectrum of BONIr.

[0099]FIG. 39. Emission spectrum of BTIr.

[0100]FIG. 40. Proton NMR spectrum of BTIr.

[0101]FIG. 41. Emission spectrum of BOIr.

[0102]FIG. 42. Proton NMR spectrum of BOIr.

[0103]FIG. 43. Emission spectrum of BTIrQ.

[0104]FIG. 44. Proton NMR spectrum of BTIrQ.

[0105]FIG. 45. Emission spectrum of BTIrP.

[0106]FIG. 46. Emission spectrum of BOIrP.

[0107]FIG. 47. Emission spectrum of btIr-type complexes with differentligands.

[0108]FIG. 48. Proton NMR spectrum of mer-Irbq.

[0109]FIG. 49. Other suitable L and X ligands for L₂MX compounds. In allof these ligands listed, one can easily substitute S for O and stillhave a good ligand.

[0110]FIG. 50. Examples of L L′ L″ M compounds. In the listed examplesof L L′ L″ M and L L′ M X compounds, the compounds would be expected toemit from the lowest energy ligand or the MLCT state, involving the bqor thpy ligands. In the listed example of an L M X X′ compound, emissiontherefrom is expected from the ppy ligand. The X and X′ ligands willmodify the physical properties (for example, a hole trapping group couldbe added to either ligand).

DETAILED DESCRIPTION OF THE INVENTION

[0111] The present invention is generally directed to emissivemolecules, which luminesce when a voltage is applied across aheterostructure of an organic light-emitting device and which moleculesare selected from the group of phosphorescent organometallic complexes,and to structures, and correlative molecules of the structures, thatoptimize the emission of the light-emitting device. The term“organometallic” is as generally understood by one of ordinary skill, asgiven, for example, in “Inorganic Chemistry” (2nd edition) by Gary L.Miessler and Donald A. Tarr, Prentice-Hall (1998). The invention isfurther directed to emissive molecules within the emissive layer of anorganic light-emitting device which molecules are comprised ofphosphorescent cyclometallated platinum, iridium or osmium complexes. Onelectroluminescence, molecules in this class may produce emission whichappears red, blue, or green. Discussions of the appearance of color,including descriptions of CIE charts, may be found in H. Zollinger,Color Chemistry, VCH Publishers, 1991 and H. J. A. Dartnall, J. K.Bowmaker, and J. D. Mollon, Proc. Roy. Soc. B (London), 1983, 220,115-130.

[0112] The present invention will now be described in detail forspecific preferred embodiments of the invention, it being understoodthat these embodiments are intended only as illustrative examples andthe invention is not to be limited thereto.

[0113] Synthesis of the Cyclometallated Platinum Complexes

[0114] We have synthesized a number of different Pt cyclometallatedcomplexes.

[0115] Numerous publications, reviews and books are dedicated to thechemistry of cyclometallated compounds, which also are calledintramolecular-coordination compounds. (I. Omae, OrganometallicIntramolecular-coordination compounds. N.Y. 1986. G. R. Newkome, W. E.Puckett, V. K. Gupta, G. E. Kiefer, Chem.Rev. 1986,86,451. A. D. Ryabov,Chem.Rev. 1990, 90, 403). Most of the publications depict mechanisticalaspects of the subject and primarily on the cyclometallated compoundswith one bi- or tri-dentate ligand bonded to metal by C-M single bondand having cycle closed with one or two other X-M bonds where X may beN, S, P, As, O. Not so much literature was devoted to bis- ortris-cyclometallated complexes, which do not possess any other ligandsbut C,N type bi-dentate ones. Some of the subject of this invention isin these compounds because they are not only expected to haveinteresting photochemical properties as most cyclometallated complexesdo, but also should exhibit increased stability in comparison with theirmonocyclometallated analogues. Most of the work on bis-cyclopaladatedand bis-cycloplatinated compounds was performed by von Zelewsky et al.(For a review see: M. Maestri, V. Balzani, Ch.Deuschel-Cornioley, A. vonZelewsky, Adv.Photochem. 1992 17, 1. L. Chassot, A. Von Zelewsky, Helv.Chim.Acta 1983, 66, 243. L. Chassot, E. Muler, A. von Zelewsky, Inorg.Chem. 1984, 23, 4249. S Bonafede, M. Ciano, F. Boletta, V. Balzani, L.Chassot, A. von Zelewsky, J Phys.Chem. 1986, 90, 3836. L. Chassot, A.von Zelewsky, D. Sandrini, M. Maestri, V. Balzani, J.Am.Chem.Soc. 1986,108, 6084. Ch.Cornioley-Deuschel, A. von Zelewsky, Inorg.Chem. 1987, 26,3354. L. Chassot, A. von Zelewsky, Inorg.Chem. 1987, 26, 2814. A. vonZelewsky, A. P. Suckling, H. Stoeckii-Evans, Inorg.Chem. 1993, 32, 4585.A. von Zelewsky, P. Belser, P. Hayoz, R. Dux, X. Hua, A. Suckling, H.Stoeckii-Evans, Coord.Chem.Rev. 1994, 132, 75. P. Jolliet, M. Gianini,A. von Zelewsky, G. Bernardinelli, H. Stoeckii-Evans, Inorg.Chem. 1996,35, 4883. H. Wiedenhofer, S. Schutzenmeier, A. von Zelewsky, H. Yersin,J.Phys. Chem. 1995, 99, 13385. M. Gianini, A. von Zelewsky, H.Stoeckii-Evans, Inorg. Chem. 1997, 36, 6094.) In one of their earlyworks, (M. Maestri, D. Sandrini, V. Balzani, L. Chassot, P. Jolliet A.von Zelewsky, Chem.Phys.Lett. 1985,122,375) luminescent properties ofthree bis-cycloplatinated complexes were investigated in detail. Thesummary of the previously reported results on Pt bis-cyclometallatedcomplexes important for our current research is as follows:

[0116] i. in general, cyclometallated complexes having a 5-membered ringformed between the metal atom and C,X ligand are more stable.

[0117] ii. from the point of view of stability of resulting compounds,complexes not containing anionic ligands are preferred; thus,bis-cyclometallated complexes are preferred to mono-cyclometallatedones.

[0118] iii. a variety of Pt(Pd) cyclometallated complexes weresynthesized, homoleptic (containing similar C,X ligands), heteroleptic(containing two different cyclometallating C,X ligands) and complexeswith one C,C cyclometallating ligand and one N,N coordinating ligand.

[0119] iv. most bis-cyclometallated complexes show M⁺ ions upon electronimpact ionization in their mass spectra; this can be a base for ourassumption on their stability upon vacuum deposition.

[0120] v. on the other hand, some of the complexes are found not to bestable in certain solvents; they undergo oxidative addition reactionsleading to Pt(IV) or Pd(IV) octahedral complexes.

[0121] vi. optical properties are reported only for some of thecomplexes; mostly absorption data is presented. Low-energy electrontransitions observed in both their absorption and emission spectra areassigned to MLCT transitions.

[0122] vii. reported luminescent properties are summarized in Table 1.Used abbreviations are explained in Scheme 1. Upon transition frombis-cyclometalated complexes with two C,N ligands to the complexes withone C,C and one N,N ligand batochromic shift in emission was observed.(M. Maestri, D. Sandrini, V. Balzani, A. von Zelewsky, C.Deuschel-Cornioley, P. Jolliet, Helv. Chim.Acia 1988, 71, 1053. TABLE 1Absorption and emission properties of several cycloplatinated complexes.Reproduced from A.von Zelewsky et. al (Chem. Phys. Lett., 1985, 122, 375and Helv. Chim. Acta 1988, 17, 1053). Abbreviation explanations aregiven in Scheme 1. emission spectra absorption 77 K 293 K solventλmax(ε) λmax(τ) λmax(τ) Pt(Phpy)₂(1) CH₃CN 402(12800) 491(4.0) —291(27700) Pt(Thpy)₂(2) CH₃CN 418(10500)  570(12.0) 578(2.2) 303(26100)Pt(Bhq)₂(3) CH₃CN 421(9200)  492(6.5) — 367(12500) 307(15000)Pt(bph)(bpy)(4)

[0123]

[0124] We synthesized different bis-cycloplatinated complexes in orderto investigate their optical properties in different hosts, bothpolymeric and molecular, and utilize them as dopants in correspondinghosts for organic light-emitting diodes (OLEDs). Usage of the complexesin molecular hosts in OLEDs prepared in the vacuum deposition processrequires several conditions to be satisfied. The complexes should besublimable and stable at the standard deposition conditions (vacuum˜10⁻⁶ torr). They should show emission properties interesting for OLEDapplications and be able to accept energy from host materials used, suchas Alq₃ or NPD. On the other hand, in order to be useful in OLEDsprepared by wet techniques, the complexes should form true solutions inconventional solvents (e.g., CHCl₃) with a wide range of concentrationsand exhibit both emission and efficient energy transfer from polymerichosts (e.g., PVK). All these properties of cycloplatinated complexeswere tested. In polymeric hosts we observe efficient luminescence fromsome of the materials.

[0125] Syntheses Proceeded as Follows:

[0126] 2-(2-thienyl)pyridine. Synthesis is shown in Scheme 2, and wasperformed according to procedure close to the published one (T.Kauffmann, A. Mitschker, A. Woltermann, Chem.Ber. 1983, 116, 992). Forpurification of the product, instead of recommended distillation, zonalsublimation was used (145-145-125° C., 2-3 hours). Light brownish whitesolid (yield 69%). Mass-spec: m/z: 237(18%), 161 (100%, M⁺), 91 (71%).¹H NMR (250 MHZ, DMSO-d₆) δ,ppm: 6.22-6.28 (d. of d., 1H), 6.70-6.80 (d.of d., 1H), 6.86-7.03 (m,3H), 7.60-7.65 (m,1H). ¹³C NMR (250 MHZ,DMSO-d₆): 118.6, 122.3, 125.2, 128.3, 128.4, 137.1, 144.6, 149.4, 151.9.

[0127] 2-(2-thienyl)quinoline. Synthesis is displayed in Scheme 3, andwas made according to published procedure (K. E. Chippendale, B. Iddon,H. Suschitzky, J.Chem.Soc. 1949, 90, 1871). Purification was madeexactly following the literature as neither sublimation nor columnchromatography did not give as good results as recrystallizations from(a) petroleum ether, and (b) EtOH-H₂O (1:1) mixture. Pale yellow solid,gets more yellow with time (yield 84%). Mass-spec: m/z: 217 (32%), 216(77%), 215 (83%), 214 (78%), 213 (77%), 212 (79%), 211(100%, M⁺), 210(93%), 209 (46%). ¹H NMR (250 MHZ, DMSO-d₆) δ,ppm: 7.18-7.24 (d. ofd.,1H), 7.48-7.58 (d. of d. of d.,1H), 7.67-7.78 (m,2H), 7.91-7.97(m,3H), 8.08-8.11 (d,1H),

[0128] 2-(2′bromophenyl)pyridine. Synthesis was performed according toliterature (D. H. Hey, C. J. M. Stirling, G. H. Williams, J.Chem. Soc.1955, 3963; R. A. Abramovich, J. G. Saha, J.Chem.Soc. 1964, 2175). It isoutlined in Scheme 4. Literature on the subject was dedicated to thestudy of aromatic substitution in different systems, including pyridine,and study of isomeric ratios in the requiting product. Thus in order toresolve isomer mixtures of different substituted phenylpyridines, not2-(2′-bromophenyl)pyridine, the authors utilized 8 ft.×{fraction (1/4)}in. column packed with ethylene glycol succinate (10%) on Chromosorb Wat 155° C. and some certain helium inlet pressure. For resolving thereaction mixture we obtained, we used column chromatography withhexanes:THF (1:1) and haxanes:THF:PrOH-1 (4:4:1) mixtures as eluents onsilica gel because this solvent mixture gave best results in TLC (threewell resolved spots). Only the first spot in the column gave mass specmajor peak corresponding to n-(2′-bromophenyl)pyridines (m/z: 233, 235),in the remaining spots this peak was minor. Mass spec of the firstfraction: m/z: 235 (97%), 233 (100%, M⁺), 154 (86%), 127 (74%). ¹H NMRof the first fraction (250 MHZ, DMSO-d6) δ, ppm: 7.27-7.51 (m,4H),7.59-7.96 (m,2H), 8.57-8.78 (m,2H).

[0129] Sublimation of the 1^(st) fraction product after column did notlead to disappearance of the peaks of contaminants in ¹H NMR spectrum,and we do not expect the sublimation to lead to resolving the isomers ifpresent.

[0130] 2-phenylpyridine. Was synthesized by literature procedure (J. C.W. Evans, C. F. H. Allen, Org. Synth. Cell. 1943, 2, 517) and isdisplayed in Scheme 5. Pale yellow oil darkening in the air (yield 48%).¹H NMR (250 MHZ, DMSO-d₆) of the product after vacuum distillation:δ,ppm: 6.70-6.76 (m,1H), 6.92-7.10 (m,3H), 7.27-7.30 (m,1H), 7.36-7.39(q,1H), 7.60-7.68 (m,2H), 8.16-8.23 (m,1H)).

[0131] 2,2′-diaminobiphenyl. Was prepared by literature method (R. E.Moore, A. Furst, J.Org. Chem. 1958, 23, 1504) (Scheme 6). Pale pinksolid (yield 69%). ¹H NMR (250 MHZ, DMSO-d₆) δ,ppm: 5.72-5.80 (t. ofd.,2H), 5.87-5.93 (d. of d., 2H), 6.03-6.09 (d. of d.,2H), 6.13-6.23 (t.of d.,2H). Mass spec: m/z: 185 (40%), 184 (100%, M⁺), 183 (73%), 168(69%), 167 (87%), 166(62%), 139 (27%).

Scheme 6: Synthesis of 2,2′-dibromobiphenyl from 2,2′-dinitrobiphenyl

[0132]

[0133] 2,2′-dibromobiphenyl. (Scheme 6) (A. Uehara, J. C. Bailar, Jr.,J.Organomet. Chem. 1982, 239,1).

[0134] 2,2′-dibromo-1,1′-binaphthyl. Was synthesized according toliterature (H. Takaya, S. Akutagawa, R. Noyori, Org.Synth. 1989, 67,20)(Scheme 7).

[0135] trans-Dichloro-bis-(diethyl sulfide) platinum (II). Prepared by apublished procedure (G. B. Kauffman, D. O. Cowan, Inorg. Synth.1953, 6,211) (Scheme 8). Bright yellow solid (yield 78%).

[0136] cis-Dichloro-bis-(diethyl sulfide)platinum (II). Prepared by apublished procedure (G. B. Kauffman, D. O. Cowan, Inorg. Synth.1953, 6,211). (Scheme 8). Yellow solid (63%).

[0137] cis-Bis[2-(2-thienyl)pyridinato-N,C^(5′) platinum (II). Wassynthesized according to literature methods (L. Chassot, A. vonZelewsky, Inorg.Chem. 1993, 32, 4585). (Scheme 9). Bright red crystals(yield 39%). Mass spec: m/z: 518 (25%), 517 (20%), 516 (81%), 513(100%,M⁺), 514 (87%), 481 (15%), 354 (23%).

[0138] cis-Bis[2-(2′-thienyl)quinolinato-N,C³) platinum (II). Wasprepared following published procedures (P. Jolliet, M. Gianini, A. vonZelewsky, G. Bernardinelli, H. Stoeckii-Evans, Inorg.Chem. 1996, 35,4883). (Scheme 10). Dark red solid (yield 21%).

[0139] Absorption spectra were recorded on AVIV Model 14DS-UV-Vis-IRspectrophotometer and corrected for background due to solventabsorption. Emission spectra were recorded on PTI QuantaMaster ModelC-60SE spectrometer with 1527 PMT detector and corrected for detectorsensitivity inhomogeneity.

[0140] Vacuum deposition experiments were performed using standard highvacuum system (Kurt J. Lesker vacuum chamber) with vacuum ˜10⁻⁶ torr.Quartz plates (ChemGlass Inc.) or borosilicate glass-IndiumTin Oxideplates (ITO, Delta Technologies,Lmtd.), if used as substrates fordeposition, were pre-cleaned according to the published procedure forthe later (A. Shoustikov, Y. You, P. E. Burrows, M. E. Thomspon, S. R.Forrest, Synth.Met. 1997, 91, 217).

[0141] Thin film spin coating experiments were done with standard spincoater (Specialty Coating Systems, Inc.) with regulatable speed,acceleration speed, and deceleration speed. Most films were spun coatwith 4000 RPM speed and maximum acceleration and deceleration for 40seconds.

[0142] Optical Properties of the Pt Cyclometalated Complexes: TABLE 1Absorption and emission properties of several cycloplatinated complexes.Reproduced from A. von Zelewsky et. al (Chem. Phys. Lett., 1985, 122,375 and Helv. Chim. Acta 1988, 71, 1053). Abbreviation explanations aregiven in Scheme 1. emission spectra absorption 77 K 293 K solventλmax(ε) λmax(τ) λmax(τ) Pt(Phpy)₂ CH₃CN 402(12800) 491(4.0) — 291(27700)Pt(Thpy)₂ CH₃CN 418(10500)  570(12.0) 578(2.2) 303(26100) Pt(Bhq)₂ CH₃CN421(9200)  492(6.5) — 367(12500) 307(15000) Pt(bph)(bpy)

[0143]

[0144] Optical Properties in Solution:

[0145] Absorbance spectra of the complexes Pt(thpy)₂, Pt(thq)₂ andPt(bph)(bpy) in solution (CHCl₃ or CH₂Cl₂) were normalized and arepresented in FIG. 1. Absorption maximum for Pt(phpy)₂ showed a maximumat ca. 400 nm, but because the complex apparently requires furtherpurification, the spectrum is not presented.

[0146] Normalized emission spectra are shown in FIG. 2. Excitationwavelengths for Pt(thpy)₂, Pt(thq)₂ and Pt(bph)(bpy) are correspondingly430 nm, 450 nm, and 449 nm (determined by maximum values in theirexcitation spectra). Pt(thpy)₂ gives strong orange to yellow emission,while Pt(thq)₂ gives two lines at 500 and 620 nm. The emission formthese materials is due to efficient phosphorescence. Pt(bph)(bpy) givesblue emission, centered at 470 nm. The emission observed forPt(bph)(bpy) is most likely due to fluorescence and not phosphorescence.

[0147] Emission lifetimes and quantum yields in solution: Pt(thPy)₂: 3.7μs (CHCl₃, deoxygenated for 10 min) 0.27 Pt(thq)₂: 2.6 μs (CHCl₃,deoxygenated for 10 min) not measured Pt(bph)(bpy): not in μs region(CH₂O₂, deoxygenated not measured for 10 min)

[0148] Optical properties in PS solid matrix:

[0149] Pt(thpy)₂: Emission maximum is at 580 nm (lifetime 6.5 μs) uponexcitation at 400 nm. Based on the increased lifetime for the sample inpolystyrene we estimate a quantum efficiency in polystyrene forPt(thpy)₂ of 0.47.

[0150] Pt(thq)₂: Emission maximum at 608 nm (lifetime 7.44 μs) uponexcitation at 450 nm.

[0151] Optical Properties of the complexes in PVK Film:

[0152] These measurements were made for Pt(thpy)₂ only.Polyvinylcarbazole (PVK) was excited at 250 nm and energy transfer fromPVK to Pt(thpy)₂ was observed (FIG. 3). The best weight PVK:Pt(thpy)₂ratio for the energy transfer was found to be ca. 100:6.3.

EXAMPLES OF LIGHT EMITTING DIODES EXAMPLE 1 ITO/PVK:PBD.Pt(thpy)₂(100:40:2)/Ag:Mg/Ag

[0153] Pt(thpy)₂ does not appear to be stable toward sublimation. Inorder to test it in an OLED we have fabricated a polymer blended OLEDwith Pt(thpy)₂ dopant. The optimal doping level was determined by thephotoluminescence study described above. The emission from this devicecomes exclusively from the Pt(thpy)₂ dopant. Typical current-voltagecharacteristic and light output curve of the device are shown in FIG. 4.Quantum efficiency dependence on applied voltage is demonstrated in FIG.5. Thus, at 22 V quantum efficiency is ca. 0.11%. The high voltagerequired to drive this device is a result of the polymer blend OLEDstructure and not the dopant. Similar device properties were observedfor a polymer blend device made with a coumarin dopant in place ofPt(thpy)₂. In addition, electroluminescence spectrum and CIE diagram areshown in FIG. 6.

EXAMPLE 2

[0154] In this example, we describe OLEDs employing the green,electrophosphorescent material fac tris(2-phenylpyridine)iridium(Ir(ppy)₃). This compound has the following formulaic representation:

[0155] The coincidence of a short triplet lifetime and reasonablephotoluminescent efficiency allows Ir(ppy)₃-based OLEDs to achieve peakquantum and power efficiencies of 8.0% (28 cd/A) and ˜30 Im/Wrespectively. At an applied bias of 4.3V, the luminance reaches 100cd/m² and the quantum and power efficiencies are 7.5% (26 cd/A) and 19Im/W, respectively.

[0156] Organic layers were deposited by high vacuum (10⁻⁶ Torr) thermalevaporation onto a cleaned glass substrate precoated with transparent,conductive indium tin oxide. A 400 A thick layer of4,4′-bis(N-(1-naphthyl)-N-phenyl-amino)biphenyl (α-NPD) is used totransport holes to the luminescent layer consisting of Ir(ppy)₃ in CBP.A 200 A thick layer of the electron transport materialtris-(8-hydroxyquinoline)aluminum (Alq₃) is used to transport electronsinto the Ir(ppy)₃:CBP layer, and to reduce Ir(ppy)₃ luminescenceabsorption at the cathode. A shadow mask with 1 mm diameter openings wasused to define the cathode consisting of a 1000 A thick layer of 25:1Mg:Ag, with a 500 A thick Ag cap. As previously (O'Brien, et al., App.Phys. Lett. 1999, 74,.442-444), we found that a thin (60 A) barrierlayer of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (bathocuproine,or BCP) inserted between the CBP and the Alq₃ was necessary to confineexcitons within the luminescent zone and hence maintain highefficiencies. In O'Brien et al., Appl. Phys. Lett. 1999, 74, 442-444, itwas argued that this layer prevents triplets from diffusing outside ofthe doped region. It was also suggested that CBP may readily transportholes and that BCP may be required to force exciton formation within theluminescent layer. In either case, the use of BCP clearly serves to trapexcitons within the luminescent region. The molecular structuralformulae of some of the materials used in the OLEDs, along with aproposed energy level diagram, is shown in FIG. 7.

[0157]FIG. 8 shows the external quantum efficiencies of severalIr(ppy)₃-based OLEDs. The doped structures exhibit a slow decrease inquantum efficiency with increasing current. Similar to the results forthe Alq₃:PtOEP system the doped devices achieve a maximum efficiency(˜8%) for mass ratios of Ir(ppy)₃:CBP of approximately 6-8%. Thus, theenergy transfer pathway in Ir(ppy)₃:CBP is likely to be similar to thatin PtOEP:Alq₃ (Baldo, et al., Nature, 1998, 395, 151; O'Brien, 1999, op.cit.) i.e. via short range Dexter transfer of triplets from the host. Atlow Ir(ppy)₃ concentrations, the lumophores often lie beyond the Dextertransfer radius of an excited Alq₃ molecule, while at highconcentrations, aggregate quenching is increased. Note thatdipole-dipole (Forster) transfer is forbidden for triplet transfer, andin the PtOEP:Alq₃ system direct charge trapping was not found to besignificant.

EXAMPLE 3

[0158] In addition to the doped device, we fabricated a heterostructurewhere the luminescent region was a homogeneous film of Ir(ppy)₃. Thereduction in efficiency (to ˜0.8% ) of neat Ir(ppy)₃ is reflected in thetransient decay, which has a lifetime of only ˜100 ns, and deviatessignificantly from mono-exponential behavior. A 6% Ir(ppy)₃:CBP devicewithout a BCP barrier layer is also shown together with a 6%Ir(ppy)₃:Alq₃ device with a BCP barrier layer. Here, very low quantumefficiencies are observed to increase with current. This behaviorsuggests a saturation of nonradiative sites as excitons migrate into theAlq₃, either in the luminescent region or adjacent to the cathode.

EXAMPLE 4

[0159] In FIG. 9 we plot luminance and power efficiency as a function ofvoltage for the device of Example 2. The peak power efficiency is ˜30lm/W with a quantum efficiency of 8%, (28 cd/A). At 100cd/m², a powerefficiency of 19 lm/W with a quantum efficiency of 7.5% (26 cd/A) isobtained at a voltage of 4.3V. The transient response of Ir(ppy)₃ in CBPis a mono-exponential phosphorescent decay of ˜500 ns, compared with ameasured lifetime (e.g., King, et al., J. Am. Chem. Soc., 1985, 107,1431-1432) of 2 μs in degassed toluene at room temperature. Theselifetimes are short and indicative of strong spin-orbit coupling, andtogether with the absence of Ir(ppy)₃ fluorescence in the transientresponse, we expect that Ir(ppy)₃ possesses strong intersystem crossingfrom the singlet to the triplet state. Thus all emission originates fromthe long lived triplet state. Unfortunately, slow triplet relaxation canform a bottleneck in electrophosphorescence and one principal advantageof Ir(ppy)₃ is that it possesses a short triplet lifetime. Thephosphorescent bottleneck is thereby substantially loosened. Thisresults in only a gradual decrease in efficiency with increasingcurrent, leading to a maximum luminance of ˜100,000 cd/m².

EXAMPLE 5

[0160] In FIG. 10, the emission spectrum and Commission Internationalede L'Eclairage (CIE) coordinates of Ir(ppy)₃ are shown for the highestefficiency device. The peak wavelength is λ=510 nm and the full width athalf maximum is 70 nm. The spectrum and CIE coordinates (x=0.27,y-0.63)are independent of current. Even at very high current densities (˜100mA/cm² ) blue emission from CBP is negligible—an indication of completeenergy transfer.

[0161] Other techniques known to one-of ordinary skill may be used inconjunction with the present invention. For example, the use of LiFcathodes (Hung, et al., Appl. Phys. Lett., 1997, 70, 152-154), shapedsubstrates (G. Gu, et al., Optics Letters, 1997, 22, 396-398), and novelhole transport materials that result in a reduction in operating voltageor increased quantum efficiency (B. Kippelen, et al., MRS (SanFrancisco, Spring, 1999) are also applicable to this work. These methodshave yielded power efficiencies of ˜20 lm/W in fluorescent smallmolecule devices (Kippelen, Id.). The quantum efficiency in thesedevices (Kido and Iizumi, App. Phys. Lett., 1998, 73, 2721) at 100 cd/m²is typically ≦4.6% (lower than that of the present invention), and hencegreen-emitting electrophosphorescent devices with power efficienciesof >40 lm/W can be expected. Purely organic materials (Hoshino andSuzuki, Appl. Phys. Lett., 1996, 69, 224-226) may sometimes possessinsufficient spin orbit coupling to show strong phosphorescence at roomtemperature. While one should not rule out the potential of purelyorganic phosphors, the preferred compounds may be transition metalcomplexes with aromatic ligands. The transition metal mixes singlet andtriplet states, thereby enhancing intersystem crossing and reducing thelifetime of the triplet excited state.

[0162] The present invention is not limited to the emissive molecule ofthe examples. One of ordinary skill may modify the organic component ofthe Ir(ppy)₃ (directly below) to obtain desirable properties.

[0163] One may have alkyl substituents or alteration of the atoms of thearomatic structure.

[0164] These molecules, related to Ir(ppy)₃, can be formed fromcommercially available ligands. The R groups can be alkyl or aryl andare preferably in the 3, 4, 7 and/or 8 positions on the ligand (forsteric reasons). The compounds should give different color emission andmay have different carrier transport rates. Thus, the modifications tothe basic Ir(ppy)₃ structure in the three molecules can alter emissiveproperties in desirable ways.

[0165] Other possible emitters are illustrated below, by way of example.

[0166] This molecule is expected to have a blue-shifted emissioncompared to Ir(ppy)₃. R and R′ can independently be alkyl or aryl.

[0167] Organometallic compounds of osmium may also be used in thisinvention. Examples include the following.

[0168] These osmium complexes will be octahedral with 6 d electrons(isoelectronic with the Ir analogs) and may have good intersystemcrossing efficiency. R and R′ are independently selected from the groupconsisting of alkyl and aryl. They are believed to be unreported in theliterature.

[0169] Herein, X can be selected from the group consisting of N or P. Rand R′ are independently selected from the group alkyl and aryl.

[0170] The molecule of the hole-transporting layer of Example 2 isdepicted below.

[0171] The present invention will work with other hole-transportingmolecules known by one of ordinary skill to work in hole transportinglayers of OLEDs.

[0172] The molecule used as the host in the emissive layer of Example 2is depicted below.

[0173] The present invention will work with other molecules known by oneof ordinary skill to work as hosts of emissive layers of OLEDs. Forexample, the host material could be a hole-transporting matrix and couldbe selected from the group consisting of substituted tri-aryl amines andpolyvinylcarbazoles.

[0174] The molecule used as the exciton blocking layer of Example 2 isdepicted below. The invention will work with other molecules used forthe exciton blocking layer, provided they meet the requirements listedin the summary of the invention.

[0175] Molecules which are suitable as components for an excitonblocking layer are not necessarily the same as molecules which aresuitable for a hole blocking layer. For example, the ability of amolecule to function as a hole blocker depends on the applied voltage,the higher the applied voltage, the less the hole blocking ability. Theability to block excitons is roughly independent of the applied voltage.

[0176] This invention is further directed to the synthesis and use ofcertain organometallic molecules of formula L₂MX which may be doped intoa host phase in an emitter layer of an organic light emitting diode.Optionally, the molecules of formula L₂MX may be used at elevatedconcentrations or neat in the emitter layer. This invention is furtherdirected to an organic light emitting device comprising an emitter layercomprising a molecule of the formula L₂MX wherein L and X areinequivalent, bidentate ligands and M is a metal, preferably selectedfrom the third row of the transition elements of the periodic table, andmost preferably Ir or Pt, which forms octahedral complexes, and whereinthe emitter layer produces an emission which has a maximum at a certainwavelength λ_(max). The general chemical formula for these moleculeswhich are doped into the host phase is L₂MX, wherein M is a transitionmetal ion which forms octahedral complexes, L is a bidentate ligand, andX is a distinct bidentate ligand. Examples of L are2-(1-naphthyl)benzoxazole)), (2-phenylbenzoxazole),(2-phenylbenzothiazole), (2-phenylbenzothiazole), (7,8-benzoquinoline),coumarin, (thienylpyridine), phenylpyridine, benzothienylpyridine,3-methoxy-2-phenylpyridine, thienylpyridine, and tolylpyridine. Examplesof X are acetylacetonate (“acac”), hexafluoroacetylacetonate,salicylidene, picolinate, and 8-hydroxyquinolinate. Further examples ofL and X are given in FIG. 49 and still further examples of L and X maybe found in Comprehensive Coordination Chemistry, Volume 2, G. Wilkinson(editor-in-chief), Pergamon Press, especially in chapter 20.1 (beginningat page 715) by M. Calligaris and L. Randaccio and in chapter 20.4(beginning at page 793) by R. S. Vagg.

[0177] Synthesis of Molecules of Formula L₂MX

[0178] The compounds of formula L₂MX can be made according to thereaction:

L₂M(μ-Cl)₂ML₂+XH→L₂MX+HCl

[0179] wherein L₂M(μ-Cl)₂ML₂ is a chloride bridged dimer with L abidentate ligand, and M a metal such as Ir; XH is a Bronsted acid whichreacts with bridging chloride and serves to introduce a bidentate ligandX, wherein XH can be, for example, acetylacetone,hexafluoroacetylacetone, 2-picolinic acid, or N-methylsalicyclanilide;and L₂MX has approximate octahedral disposition of the bidentate ligandsL, L, and X about M.

[0180] L₂Ir(μ-Cl)₂IrL₂ complexes were prepared from IrCl₃.nH₂O and theappropriate ligand by literature procedures (S. Sprouse, K. A. King, P.J. Spellane, R. J. Watts, J. Am. Chem. Soc., 1984, 106, 6647-6653; forgeneral reference: G. A. Carlson, et al., Inorg. Chem., 1993, 32, 4483;B. Schmid, et al., Inorg. Chem., 1993, 33, 9; F. Garces, et al.; Inorg.Chem., 1988, 27, 3464; M. G. Colombo, et al., Inorg. Chem., 1993, 32,3088; A. Mamo, et al., Inorg. Chem., 1997, 36, 5947; S. Serroni, et al.;J. Am. Chem. Soc., 1994, 116, 9086; A. P. Wilde, et al., J. Phys. Chem.,1991, 95, 629; J. H. van Diemen, et al., Inorg. Chem., 1992, 31, 3518;M. G. Colombo, et al., Inorg. Chem., 1994, 33, 545), as described below.

[0181] Ir(3-MeOppy)₃. Ir(acac)₃ (0.57 g, 1.17 mmol) and3-methoxy-2-phenylpyridine (1.3 g, 7.02 mmol) were mixed in 30 ml ofglycerol and heated to 200° C. for 24 hrs under N₂. The resultingmixture was added to 100 ml of 1 M HCl. The precipitate was collected byfiltration and purified by column chromatography using CH₂Cl₂ as theeluent to yield the product as bright yellow solids (0.35 g, 40%). MS(EI): m/z (relative intensity) 745 (M⁺, 100), 561 (30), 372 (35).Emission spectrum in FIG. 17.

[0182] tpyIrsd. The chloride bridge dimer (tpyIrCl)₂ (0.07 g, 0.06mmol), salicylidene (0.022 g, 0.16 mmol) and Na₂CO₃ (0.02 g, 0.09 mmol)were mixed in 10 ml of 1,2-dichloroethane and 2 ml of ethanol. Themixture was refluxed under N₂ for 6 hrs or until no dimer was revealedby TLC. The reaction was then cooled and the solvent evaporated. Theexcess salicylidene was removed by gentle heating under vacuum. Theresidual solid was redissolved in CH₂Cl₂ and the insoluble inorganicmaterials were removed by filtration. The filtrate was concentrated andcolumn chromatographed using CH₂Cl₂ as the eluent to yield the productas bright yellow solids (0.07 g, 85%). MS (EI): m/z (relative intensity)663 (M⁺, 75), 529 (100), 332 (35). The emission spectrum is in FIG. 18and the proton NMR spectrum is in FIG. 19.

[0183] thpyIrsd. The chloride bridge dimer (thpyIrCl)₂ (0.21 g, 0.19mmol) was treated the same way as (tpyIrCl)₂. Yield: 0.21 g, 84%. MS(EI): m/z (relative intensity) 647 (M⁺, 100), 513 (30), 486 (15), 434(20), 324 (25). The emission spectrum is in FIG. 20 and the proton NMRspectrum is in FIG. 21.

[0184] btIrsd. The chloride bridge dimer (btIrCl)₂ (0.05 g, 0.039 mmol)was treated the same way as (tpyIrCl)₂. Yield: 0.05 g, 86%. MS (EI): m/z(relative intensity) 747 (M⁺, 100), 613 (100), 476 (30), 374 (25), 286(32). The emission spectrum is in FIG. 22 and the proton NMR spectrum isin FIG. 23.

[0185] Ir(bq)₂(acac), BQIr. The chloride bridged dimer (Ir(bq)₂Cl)₂(0.091 g, 0.078 mmol), acetylacetone (0.021 g) and sodium carbonate(0.083 g) were mixed in 10 ml of 2-ethoxyethanol. The mixture wasrefluxed under N₂ for 10 hrs or until no dimer was revealed by TLC. Thereaction was then cooled and the yellow precipitate filtered. Theproduct was purified by flash chromatography using dichloromethane.Product: bright yellow solids (yield 91%). ¹H NMR (360 MHz, acetone-d₆),ppm: 8.93 (d,2H), 8.47 (d,2H), 7.78 (m,4H), 7.25 (d,2H), 7.15 (d,2H),6.87 (d,2H), 6.21 (d,2H), 5.70 (s,1H), 1.63 (s,6H). MS, e/z: 648(M+,80%), 549 (100%). The emission spectrum is in FIG. 24 and the protonNMR spectrum is in FIG. 25.

[0186] Ir(bq)₂(Facac), BQIrFA. The chloride bridged dimer (Ir(bq)₂Cl)₂(0.091 g, 0.078 mmol), hexafluoroacetylacetone (0.025 g) and sodiumcarbonate (0.083 g) were mixed in 10 ml of 2-ethoxyethanol. The mixturewas refluxed under N₂ for 10 hrs or until no dimer was revealed by TLC.The reaction was then cooled and the yellow precipitate filtered. Theproduct was purified by flash chromatography using dichloromethane.Product: yellow solids (yield 69%). ¹H NMR (360 MHz, acetone-d₆), ppm:8.99 (d,2H), 8.55 (d,2H), 7.86 (m,4H), 7.30 (d,2H), 7.14 (d,2H), 6.97(d,2H), 6.13 (d,2H), 5.75 (s,1H). MS, e/z: 684 (M+,59%), 549 (100%).Emission spectrum in FIG. 26.

[0187] Ir(thpy)₂(acac), THPIr. The chloride bridged dimer (Ir(thpy)₂Cl)₂(0.082 g, 0.078 mmol), acetylacetone (0.025 g) and sodium carbonate(0.083 g) were mixed in 10 ml of 2-ethoxyethanol. The mixture wasrefluxed under N₂ for 10 hrs or until no dimer was revealed by TLC. Thereaction was then cooled and the yellow precipitate filtered. Theproduct was purified by flash chromatography using dichloromethane.Product: yellow-orange solid (yield 80%). ¹H NMR (360 MHz, acetone-d₆),ppm: 8.34 (d,2H), 7.79 (m,2H), 7.58 (d,2H), 7.21 (d,2H), 7.15 (d,2H),6.07 (d,2H), 5.28 (s,1H), 1.70 (s,6H). MS, e/z: 612 (M+,89%), 513(100%). The emission spectrum is in FIG. 27 (noted “THIr”) and theproton NMR spectrum is in FIG. 28.

[0188] Ir(ppy)₂(acac), PPIr. The chloride bridged dimer (Ir(ppy)₂Cl)₂(0.080 g, 0.078 mmol), acetylacetone (0.025 g) and sodium carbonate(0.083 g) were mixed in 10 ml of 2-ethoxyethanol. The mixture wasrefluxed under N₂ for 10 hrs or until no dimer was revealed by TLC. Thereaction was then cooled and the yellow precipitate filtered. Theproduct was purified by flash chromatography using dichloromethane.Product: yellow solid (yield 87%). ¹H NMR (360 MHz, acetone-d₆), ppm:8.54 (d,2H), 8.06 (d,2H), 7.92 (m,2H), 7.81 (d,2H), 7.35 (d,2H), 6.78(m,2H), 6.69 (m,2H), 6.20 (d,2H), 5.12 (s,1H), 1.62 (s,6H). MS, e/z: 600(M+,75%), 501 (100%). The emission spectrum is in FIG. 29 and the protonNMR spectrum is in FIG. 30.

[0189] Ir(bthpy)₂(acac), BTPIr. The chloride bridged dimer(Ir(bthpy)₂Cl)₂ (0.103 g, 0.078 mmol), acetylacetone (0.025 g) andsodium carbonate (0.083 g) were mixed in 10 ml of 2-ethoxyethanol. Themixture was refluxed under N₂ for 10 hrs or until no dimer was revealedby TLC. The reaction was then cooled and the yellow precipitatefiltered. The product was purified by flash chromatography usingdichloromethane. Product: yellow solid (yield 49%). MS, e/z: 712(M+,66%), 613 (100%). Emission spectrum is in FIG. 31.

[0190] [Ir(ptpy)₂Cl]₂. A solution of IrCl₃.xH₂O (1.506 g, 5.030 mmol)and 2-(p-tolyl)pyridine (3.509 g, 20.74 mmol) in 2-ethoxyethanol (30 mL)was refluxed for 25 hours. The yellow-green mixture was cooled to roomtemperature and 20 mL of 1.0 M HCl was added to precipitate the product.The mixture was filtered and washed with 100 mL of 1.0 M HCl followed by50 mL of methanol then dried. The product was obtained as a yellowpowder (1.850 g, 65%).

[0191] [Ir(ppz)₂Cl]₂. A solution of IrCl₃.xH2O (0.904 g, 3.027 mmol) and1-phenylpyrazole (1.725 g, 11.96 mmol) in 2-ethoxyethanol (30 mL) wasrefluxed for 21 hours. The gray-green mixture was cooled to roomtemperature and 20 mL of 1.0 M HCl was added to precipitate the product.The mixture was filtered and washed with 100 mL of 1.0 M HCl followed by50 mL of methanol then dried. The product was obtained as a light graypowder (1.133 g, 73%).

[0192] [Ir(C6)₂Cl]₂. A solution of IrCl₃.xH₂O (0.075 g, 0.251 mmol) andcoumarin C6 [3-(2-benzothiazolyl)-7-(diethyl)coumarin] (Aldrich) (0.350g, 1.00 mmol) in 2-ethoxyethanol (15 mL) was refluxed for 22 hours. Thedark red mixture was cooled to room temperature and 20 mL of 1.0 M HClwas added to precipitate the product. The mixture was filtered andwashed with 100 mL of 1.0 M HCl followed by 50 mL of methanol. Theproduct was dissolved in and precipitated with methanol. The solid wasfiltered and washed with methanol until no green emission was observedin the filtrate. The product was obtained as an orange powder (0.0657 g,28%).

[0193] Ir(ptpy)₂(acac) (tpyIr). A solution of [Ir(ptpy)₂Cl]₂ (1.705 g,1.511 mmol), 2,4-pentanedione (3.013 g, 30.08 mmol) and (1.802 g, 17.04mmol) in 1,2-dichloroethane (60 mL) was refluxed for 40 hours. Theyellow-green mixture was cooled to room temperature and the solvent wasremoved under reduced pressure. The product was taken up in 50 mL ofCH₂Cl₂ and filtered through Celite. The solvent was removed underreduced pressure to yield orange crystals of the product (1.696 g, 89%).The emission spectrum is given in FIG. 32. The results of an x-raydiffraction study of the structure are given in FIG. 33. One sees thatthe nitrogen atoms of the tpy (“tolyl pyridyl”) groups are in a transconfiguration. For the x-ray study, the number of reflections was 4663and the R factor was 5.4%.

[0194] Ir(C6)₂(acac) (C6Ir). Two drops of 2,4-pentanedione and an excessof Na₂CO₃ was added to solution of [Ir(C6)₂Cl]₂ in CDCl₃. The tube washeated for 48 hours at 50° C. and then filtered through a short plug ofCelite in a Pasteur pipet. The solvent and excess 2,4-pentanedione wereremoved under reduced pressure to yield the product as an orange solid.Emission of C6 in FIG. 34 and of C6Ir in FIG. 35.

[0195] Ir(ppz)₂picolinate (PZIrp). A solution of [Ir(Ppz)₂Cl]₂ (0.0545g, 0.0530 mmol) and picolinic acid (0.0525 g, 0.426 mmol) in CH₂Cl₂ (15mL) was refluxed for 16 hours. The light green mixture was cooled toroom temperature and the solvent was removed under reduced pressure. Theresultant solid was taken up in 10 mL of methanol and a light greensolid precipitated from the solution. The supernatant liquid wasdecanted off and the solid was dissolved in CH₂Cl₂ and filtered througha short plug of silica. The solvent was removed under reduced pressureto yield light green crystals of the product (0.0075 g, 12%). Emissionin FIG. 36.

[0196] 2-(1-naphthyl)benzoxazole, (BZO-Naph). (11.06 g, 101 mmol) of2-aminophenol was mixed with (15.867 g, 92.2 mmol) of 1-naphthoic acidin the presence of polyphosphoric acid. The mixture was heated andstirred at 240° C. under N₂ for 8 hrs. The mixture was allowed to coolto 100° C., this was followed by addition of water. The insolubleresidue was collected by filtration, washed with water then reslurriedin an excess of 10% Na₂CO₃. The alkaline slurry was filtered and theproduct washed thoroughly with water and dried under vacuum. The productwas purified by vacuum distillation. BP 140° C./0.3 mmHg. Yield 4.8 g(21%).

[0197] Tetrakis(2-(1-naphthyl)benzoxazoleC²,N′)(μ-dichloro)diiridium.((Ir₂(BZO-Naph)₄Cl)₂). Iridium trichloride hydrate (0.388 g) wascombined with 2-(1-naphthyl)benzoxazole (1.2 g, 4.88 mmol). The mixturewas dissolved in 2-ethoxyethanol (30 mL) then refluxed for 24 hrs. Thesolution was cooled to room temperature, the resulting orange solidproduct was collected in a centrifuge tube. The dimer was washed withmethanol followed by chloroform through four cycles ofcentrifuge/redispersion cycles. Yield 0.66 g.

[0198] Bis(2-(1-naphthyl)benzoxazole)acetylacetonate,Ir(BZO-Naph)₂(acac), (BONIr). The chloride bridged dimer(Ir₂(BZO-Naph)₄Cl)₂ (0.66 g, 0.46 mmol), acetylacetone (0.185 g) andsodium carbonate (0.2 g) were mixed in 20 ml of dichloroethane. Themixture was refluxed under N₂ for 60 hrs. The reaction was then cooledand the orange/red precipitate was collected in centrifuge tube. Theproduct was washed with water/methanol (1:1) mixture followed bymethanol wash through four cycles of centrifuge/redispersion cycles. Theorange/red solid product was purified by sublimation. SP 250° C./2×10⁻⁵torr, yield 0.57 g (80%). The emission spectrum is in FIG. 37 and theproton NMR spectrum is in FIG. 38.

[0199] Bis(2-phenylbenzothiazole)Iridium acetylacetonate (BTIr). 9.8mmol (0.98 g, 1.0 mL) of 2,4-pentanedione was added to aroom-temperature solution of 2.1 mmol 2-phenylbenzothiazole Iridiumchloride dimer (2.7 g) in 120 mL of 2-ethoxyethanol. Approximately 1 gof sodium carbonate was added, and the mixture was heated to refluxunder nitrogen in an oil bath for several hours. Reaction mixture wascooled to room temperature, and the orange precipitate was filtered offvia vacuum. The filtrate was concentrated and methanol was added toprecipitate more product. Successive filtrations and precipitationsafforded a 75% yield. The emission spectrum is in FIG. 39 and the protonNMR spectrum is in FIG. 40.

[0200] Bis(2-phenylbenzooxazole)Iridium acac (BOIr). 9.8 mmol (0.98 g,1.0 mL) of 2,4-pentanedione was added to a room-temperature solution of2.4 mmol 2-phenylbenzoxazole Iridium chloride dimer (3.0 g) in 120 mL of2-ethoxyethanol. Approximately 1 g of sodium carbonate was added, andthe mixture was heated to reflux under nitrogen in an oil bath overnight(˜16 hrs.). Reaction mixture was cooled to room temperature, and theyellow precipitate was filtered off via vacuum. The filtrate wasconcentrated and methanol was added to precipitate more product.Successive filtrations and precipitations afforded a 60% yield. Theemission spectrum is in FIG. 41 and the proton NMR spectrum is in FIG.42.

[0201] Bis(2-phenylbenzothiazole)Iridium (8-hydroxyquinolate) (BTIrQ).4.7 mmol (0.68 g) of 8-hydroxyquinoline was added to a room-temperaturesolution of 0.14 mmol 2-phenylbenzothiazole Iridium chloride dimer (0.19g) in 20 mL of 2-ethoxyethanol. Approximately 700 mg of sodium carbonatewas added, and the mixture was heated to reflux under nitrogen in an oilbath overnight (23 hrs.). Reaction mixture was cooled to roomtemperature, and the red precipitate was filtered off via vacuum. Thefiltrate was concentrated and methanol was added to precipitate moreproduct. Successive filtrations and precipitations afforded a 57% yield.The emission spectrum is in FIG. 43 and the proton NMR spectrum is inFIG. 44.

[0202] Bis(2-phenylbenzothiazole)Iridium picolinate (BTIrP). 2.14 mmol(0.26 g) of picolinic acid was added to a room-temperature solution of0.80 mmol 2-phenylbenzothiazole Iridium chloride dimer (1.0 g) in 60 mLof dichloromethane. The mixture was heated to reflux under nitrogen inan oil bath for 8.5 hours. The reaction mixture was cooled to roomtemperature, and the yellow precipitate was filtered off via vacuum. Thefiltrate was concentrated and methanol was added to precipitate moreproduct. Successive filtrations and precipitations yielded about 900 mgof impure product. Emission spectrum is in FIG. 45.

[0203] Bis(2-phenylbenzooxazole)Iridium picolinate (BOIrP). 0.52 mmol(0.064 g) of picolinic acid was added to a room-temperature solution of0.14 mmol 2-phenylbenzoxazole Iridium chloride dimer (0.18 g) in 20 mLof dichloromethane. The mixture was heated to reflux under nitrogen inan oil bath overnight (17.5 hrs.). Reaction mixture was cooled to roomtemperature, and the yellow precipitate was filtered off via vacuum. Theprecipitate was dissolved in dichloromethane and transferred to a vial,and the solvent was removed. Emission spectrum is in FIG. 46.

[0204] Comparative emission spectra for different L′ in btIr complexesare shown in FIG. 47.

[0205] These syntheses just discussed have certain advantages over theprior art. Compounds of formula PtL₃ cannot be sublimed withoutdecomposition. Obtaining compounds of formula IrL₃ can be problematic.Some ligands react cleanly with Ir(acac)₃ to give the tris complex, butmore than half of the ligands we have studied do not react cleanly inthe reaction:

3 L+Ir(acac)₃→L₃Ir+(acac)H;

[0206] typically 30% yield, L=2-phenytpyridine, benzoquinoline,2-thienylpyridine. A preferred route to Ir complexes can be through thechloride-bridged dimer L₂M(μ-Cl)₂ML₂ via the reaction:

4 L+IrCl₃.nH₂O→L₂M(μ-Cl)₂ML₂+4 HCl

[0207] Although fewer than 10% of the ligands we have studied failed togive the Ir dimer cleanly and in high yield, the conversion of the dimerinto the tris complex IrL₃ is problematic working for only a fewligands. L₂M(μ-Cl)₂ML₂+2Ag′+2L→L₃Ir+2AgCl.

[0208] We have discovered that a far more fruitful approach to preparingphosphorescent complexes is to use chloride bridged dimers to createemitters. The dimer itself does not emit strongly, presumably because ofstrong self quenching by the adjacent metal (e.g., iridium) atoms. Wehave found that the chloride ligands can be replaced by a chelatingligand to give a stable, octahedral metal complex through the chemistry:

L₂M(μ-Cl)₂ML₂+XH→L₂MX+HCl

[0209] We have extensively studied the system wherein M=iridium. Theresultant iridium complexes emit strongly, in most cases with lifetimesof 1-3 microseconds (“μsec”). Such a lifetime is indicative ofphosphorescence (see Charles Kittel, Introduction to Solid StatePhysics). The transition in these materials is a metal ligand chargetransfer (“MLCT”).

[0210] In the discussion that follows below, we analyze data of emissionspectra and lifetimes of a number of different complexes, all of whichcan be characterized as L₂MX (M=Ir), where L is a cyclometallated(bidentate) ligand and X is a bidentate ligand. In nearly every case,the emission in these complexes is based on an MLCT transition betweenIr and the L ligand or a mixture of that transition and an intraligandtransition. Specific examples are described below. Based on theoreticaland spectroscopic studies, the complexes have an octahedral coordinationabout the metal (for example, for the nitrogen heterocycles of the Lligand, there is a trans disposition in the Ir octahedron).Specifically, in FIG. 11, we give the structure for L₂IrX, whereinL=2-phenyl pyridine and X=acac, picolinate (from picolinic acid),salicylanilide, or 8-hydroxyquinolinate.

[0211] A slight variation of the synthetic route to make L₂IrX allowsformation of meridianal isomers of formula L₃Ir. The L₃Ir complexes thathave been disclosed previously all have a facial disposition of thechelating ligands. Herewith, we disclose the formation and use ofmeridianal L₃Ir complexes as phosphors in OLEDs. The two structures areshown in FIG. 12.

[0212] The facial L₃Ir isomers have been prepared by the reaction of Lwith Ir(acac)₃ in refluxing glycerol as described in equation 2 (below).A preferred route into L₃Ir complexes is through the chloride bridgeddimer (L₂Ir(μ-Cl)₂IrL₂), equation 3+4 (below). The product of equation 4is a facial isomer, identical to the one formed from Ir(acac)₃. Thebenefit of the latter prep is a better yield of facial-L₃Ir. If thethird ligand is added to the dimer in the presence of base andacetylacetone (no Ag⁺), a good yield of the meridianal isomer isobtained. The meridianal isomer does not convert to the facial one onrecrystallization, refluxing in coordinating solvents or on sublimation.Two examples of these meridianal complexes have been formed, mer-Irppyand mer-Irbq (FIG. 13); however, we believe that any ligand that gives astable facial-L₃Ir can be made into a meridianal form as well.

3 L+Ir(acac)₃→facial-L₃Ir+acacH   (1)

[0213] typically 30% yield, L=2-phenylpyridine, bezoquinoline,2-thienylpyridine

4 L+IrCl₃.nH₂O→L₂Ir(μ-Cl)₂IrL₂+4 HCl

[0214] typically >90% yield, see attached spectra for examples of L,also works well for all ligands that work in equation (2)

L₂Ir(μ-Cl)₂IrL₂+2 Ag⁺+2 L→2 facial-L₃Ir+2 AgCl

[0215] typically 30% yield, only works well for the same ligands thatwork well for equation (2)

L₂Ir(μ-Cl)₂IrL₂+XH+Na₂CO₃+L→merdianal-L₃Ir

[0216] typically >80% yield, XH=acetylacetone

[0217] Surprisingly, the photophysics of the meridianal isomers isdifferent from that of the facial forms. This can be seen in the detailsof the spectra discussed below, which show a marked red shift andbroadening in the meridianal isomer relative to its facial counterpart.The emission lines appear as if a red band has been added to the bandcharacteristic of the facial-L₃Ir. The structure of the meridianalisomer is similar to those of L₂IrX complexes, with respect, forexample, to the arrangement of the N atoms of the ligands about Ir.Specifically, for L=ppy ligands, the nitrogen of the L ligand is transin both mer-Ir(ppy)₃ and in (ppy)₂Ir(acac) further, one of the L ligandsfor the mer-L₃Ir complexes has the same coordination as the X ligand ofL₂IrX complexes. In order to illustrate this point a model ofmer-Ir(ppy)₃ is shown next to (Ppy)₂Ir(acac) in FIG. 14. One of the ppyligands of mer-Ir(ppy)₃ is coordinated to the Ir center in the samegeometry as the acac ligand of (ppy)₂Ir(acac).

[0218] The HOMO and LUMO energies of these L₃Ir molecules are clearlyaffected by the choice of isomer. These energies are very important iscontrolling the current-voltage characteristics and lifetimes of OLEDsprepared with these phosphors. The syntheses for the two isomersdepicted in FIG. 13 are as follows.

[0219] Syntheses of Meridianal Isomers

[0220] mer-Irbq: 91 mg (0.078 mmol) of [Ir(bq)₂Cl]₂ dimer, 35.8 mg (0.2mmol) of 7,8-benzoquinoline, 0.02 ml of acetylacetone (ca. 0.2 mmol) and83 mg (0.78 mmol) of sodium carbonate were boiled in 12 ml of2-ethoxyethanol (used as received) for 14 hours in inert atmosphere.Upon cooling yellow-orange precipitate forms and is isolated byfiltration and flash chromatography (silica gel, CH₂Cl₂) (yield 72%). 1HNMR (360 MHz, dichloromethane-d2), ppm: 8.31 (q,1H), 8.18 (q,1H), 8.12(q,1H), 8.03(m,2H), 7.82 (m, 3H), 7.59 (m,2H), 7.47 (m,2H), 7.40 (d,1H),7.17 (m,9), 6.81 (d,1H), 6.57 (d,1H). MS, e/z: 727 (100%, M+). NMRspectrum in FIG. 48.

[0221] mer-Ir(tpy)₃: A solution of IrCl₃.xH₂O (0.301 g, 1.01 mmol),2-(p-tolyl)pyridine (1.027 g, 6.069 mmol), 2,4-pentanedione (0.208 g,2.08 mmol) and Na₂CO₃ (0.350 g, 3.30 mmol) in 2-ethoxyethanol (30 mL)was refluxed for 65 hours. The yellow-green mixture was cooled to roomtemperature and 20 mL of 1.0 M HCl was added to precipitate the product.The mixture was filtered and washed with 100 mL of 1.0 M HCl followed by50 mL of methanol then dried and the solid was dissolved in CH₂Cl₂ andfiltered through a short plug of silica. The solvent was removed underreduced pressure to yield the product as a yellow-orange powder (0.265g, 38%).

[0222] This invention is further directed toward the use of theabove-noted dopants in a host phase. This host phase may be comprised ofmolecules comprising a carbazole moiety. Molecules which fall within thescope of the invention are included in the following.

[0223] [A line segment denotes possible substitution at any availablecarbon atom or atoms of the indicated ring by alkyl or aryl groups.]

[0224] An additional preferred molecule with a carbazole functionalityis 4,4′-N,N′-dicarbazole-biphenyl (CBP), which has the formula:

[0225] The light emitting device structure that we chose to use is verysimilar to the standard vacuum deposited one. As an overview, a holetransporting layer (“HTL”) is first deposited onto the ITO (indium tinoxide) coated glass substrate. For the device yielding 12% quantumefficiency, the HTL consisted of 30 nm (300 Å) of NPD. Onto the NPD athin film of the organometallic compound doped into a host matrix isdeposited to form an emitter layer. In the example, the emitter layerwas CBP with 12% by weight bis(2-phenylbenzothiazole)iridiumacetylacetonate (termed “BTIr”), and the layer thickness was 30 nm (300Å). A blocking layer is deposited onto the emitter layer. The blockinglayer consisted of bathcuproine (“BCP”), and the thickness was 20 nm(200 Å). An electron transport layer is deposited onto the blockinglayer. The electron transport layer consisted of Alq₃ of thickness 20nm. The device is finished by depositing a Mg—Ag electrode onto theelectron transporting layer. This was of thickness 100 nm. All of thedepositions were carried out at a vacuum less than 5×10⁻⁵ Torr. Thedevices were tested in air, without packaging.

[0226] When we apply a voltage between the cathode and the anode, holesare injected from ITO to NPD and transported by the NPD layer, whileelectrons are injected from MgAg to Alq and transported through Alq andBCP. Then holes and electrons are injected into EML and carrierrecombination occurs in CBP, the excited states were formed, energytransfer to BTIr occurs, and finally BTIr molecules are excited anddecay radiatively.

[0227] As illustrated in FIG. 15, the quantum efficiency of this deviceis 12% at a current density of about 0.01 mA/cm². Pertinent terms are asfollows: ITO is a transparent conducting phase of indium tin oxide whichfunctions as an anode; ITO is a degenerate semiconductor formed bydoping a wide band semiconductor; the carrier concentration of the ITOis in excess of 10¹⁹/cm³; BCP is an exciton blocking and electrontransport layer; Alq₃ is an electron injection layer; other holetransport layer materials could be used, for example, TPD, a holetransport layer, can be used.

[0228] BCP functions as an electron transport layer and as an excitonblocking layer, which layer has a thickness of about 10 nm (100 Å). BCPis 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (also calledbathocuproine) which has the formula:

[0229] The Alq₃, which functions as an electron injection/electrontransport layer has the following formula:

[0230] In general, the doping level is varied to establish the optimumdoping level.

[0231] As noted above, fluorescent materials have certain advantages asemitters in devices. If the L ligand that is used in making the L₂MX(for example, M=Ir) complex has a high fluorescent quantum efficiency,it is possible to use the strong spin orbit coupling of the Ir metal toefficiently intersystem cross in and out of the triplet states of theligands. The concept is that the Ir makes the L ligand an efficientphosphorescent center. Using this approach, it is possible to take anyfluorescent dye and make an efficient phosphorescent molecule from it(that is, L fluorescent but L₂MX (M=Ir) phosphorescent).

[0232] As an example, we prepared a L₂IrX wherein L=coumarin and X=acac.We refer to this as coumarin-6 [“C6Ir”]. The complex gives intenseorange emission, whereas coumarin by itself emits green. Both coumarinand C6Ir spectra are given in the Figures.

[0233] Other fluorescent dyes would be expected to show similar spectralshifts. Since the number of fluorescent dyes that have been developedfor dye lasers and other applications is quite large, we expect thatthis approach would lead to a wide range of phosphorescent materials.

[0234] One needs a fluorescent dye with suitable functionality such thatit can be metallated by the metal (for example, iridium) to make a 5- or6-membered metallocycle. All of the L ligands we have studied to datehave sp² hybridized carbons and heterocyclic N atoms in the ligands,such that one can form a five membered ring on reacting with Ir.

[0235] Potential degradation reactions, involving holes or electrons,can occur in the emitter layer. The resultant oxidation or reduction canalter the emitter, and degrade performance. In order to get the maximumefficiency for phosphor doped OLEDs, it is important to control theholes or electrons which lead to undesirable oxidation or reductionreactions. One way to do this is to trap carriers (holes or electrons)at the phosphorescent dopant. It may be beneficial to trap the carrierat a position remote from the atoms or ligands responsible for thephosphorescence. The carrier that is thus remotely trapped could readilyrecombine with the opposite carrier either intramolecularly or with thecarrier from an adjacent molecule.

[0236] An example of a phosphor designed to trap holes is shown in FIG.16. The diarylamine group on the salicylanlide group is expected to havea HOMO level 200-300 mV above that of the Ir complex (based onelectrochemical measurements), leading to the holes being trappedexclusively at the amine groups. Holes will be readily trapped at theamine, but the emission from this molecule will come from MLCT andintraligand transitions from the Ir(phenylpyridine) system. An electrontrapped on this molecule will most likely be in one of the pyridylligands. Intramolecular recombination will lead to the formation of anexciton, largely in the Ir(phenylpyridine) system. Since the trappingsite is on the X ligand, which is typically not involved extensively inthe luminescent process, the presence of the trapping site will notgreatly affect the emission energy for the complex. Related moleculescan be designed in which electron carriers are trapped remoted to theL₂Ir system.

[0237] As found in the IrL₃ system, the emission color is stronglyaffected by the L ligand. This is consistent with the emission involvingeither MLCT or intraligand transitions. In all of the cases that we havebeen able to make both the tris complex (i.e., IrL₃ ) and the L₂IrXcomplex, the emission spectra are very similar. For example Ir(ppy)₃ and(ppy)₂Ir(acac) (acronym=PPIr) give strong green emission with a λ_(max)of 510 nm. A similar trend is seen in comparing Ir(BQ)₃ and Ir(thpy)₃ totheir L₂Ir(acac) derivatives, i.e., in some cases, no significant shiftin emission between the two complexes.

[0238] However, in other cases, the choice of X ligand affects both theenergy of emission and efficiency. Acac and salicylanilide L₂IrXcomplexes give very similar spectra. The picolinic acid derivatives thatwe have prepared thus far show a small blue shift (15 nm) in theiremission spectra relative to the acac and salicylanilide complexes ofthe same ligands. This can be seen in the spectra for BTIr, BTIrsd andBTIrpic. In all three of these complexes we expect that the emissionbecomes principally form MLCT and Intra-L transitions and the picolinicacid ligands are changing the energies of the metal orbitals and thusaffecting the MLCT bands.

[0239] If an X ligand is used whose triplet levels fall lower in energythan the “L₂Ir” framework, emission from the X ligand can be observed.This is the case for the BTIRQ complex. In this complex the emissionintensity is very weak and centered at 650 nm. This was surprising sincethe emission for the BT ligand based systems are all near 550 nm. Theemission in this case is almost completely from Q based transitions. Thephosphorescence spectra for heavy metal quinolates (e.g., IrQ₃ or PtQ₂)are centered at 650 nm. The complexes themselves emit with very lowefficiency, <0.01. Both the energy and efficiency of the L₂IrQ materialis consistent “X” based emission. If the emission from the X ligand orthe “IrX” system were efficient this could have been a good red emitter.It is important to note that while all of the examples listed here arestrong “L” emitters, this does not preclude a good phosphor from beingformed from “X” based emission.

[0240] The wrong choice of X ligand can also severally quench theemission from L₂IrX complexes. Both hexafluoro-acac and diphenyl-acaccomplexes give either very weak emission of no emission at all when usedas the X ligand in L₂IrX complexes. The reasons why these ligands quenchemission so strong are not at all clear, one of these ligands is moreelectron withdrawing than acac and the other more electron donating. Wegive the spectrum for BQIrFA in the Figures. The emission spectrum forthis complex is slightly shifted from BQIr, as expected for the muchstronger electron withdrawing nature of the hexafluoroacac ligand. Theemission intensity from BQIrFA is at least 2 orders of magnitude weakerthan BQIr. We have not explored the complexes of these ligands due tothis severe quenching problem.

[0241] CBP was used in the device described herein. The invention willwork with other hole-transporting molecules known by one of ordinaryskill to work in hole transporting layers of OLEDs. Specifically, theinvention will work with other molecules comprising a carbazolefunctionality, or an analogous aryl amine functionality.

[0242] The OLED of the present invention may be used in-substantiallyany type of device which is comprised of an OLED, for example, in OLEDsthat are incorporated into a larger display, a vehicle, a computer, atelevision, a printer, a large area wall, theater or stadium screen, abillboard or a sign.

1-90. (canceled)
 91. A composition comprising a compound of formulaL₂MX, wherein L and X are inequivalent monoanionic bidentate ligands, Mis a metal that forms octahedral complexes, and the L ligands arecoordinated to M through an sp² hybridized carbon and a heteroatom. 92.The composition of claim 91 wherein the heteroatoms of the L ligands arein a trans configuration.
 93. The composition of claim 91 wherein theheteroatom of the L ligand is nitrogen.
 94. The composition of claim 91wherein the X ligand is an O—O ligand.
 95. The composition of claim 91wherein the X ligand is an N—O ligand.
 96. The composition of claim 91wherein M is selected from the group consisting of the transition metalsof the third row of the transition series of the periodic table.
 97. Thecomposition of claim 91 wherein M comprises Pt.
 98. The composition ofclaim 91 wherein M comprises Ir.
 99. The composition of claim 91 whereinthe ring comprising the metal M, the sp² hybridized carbon and theheteroatom consists of five atoms.
 100. The composition of claim 91wherein the ring comprising the metal M, the sp² hybridized carbon andthe heteroatom consists of six atoms.
 101. The composition of claim 91wherein L is selected from the group consisting of a2-(1-naphthyl)benzoxazole, a 2-phenylbenzoxazole, a2-phenylbenzothiazole, a 7,8-benzoquinoline, a coumarin, aphenylpyridine, a benzothienylpyridine, a 3-methoxy-2-phenylpyridine, athienylpyridine, and a tolylpyridine.
 102. The composition of claim 91wherein the X ligand is selected from the group consisting of anacetylacetonate, a hexafluoroacetylacetonate, a salicylidene, apicolinate, and a 8-hydroxyquinolinate.
 103. The composition of claim 91wherein the L ligand is a substituted or unsubstituted ligand selectedfrom the group consisting of a phenylimine, a vinylpyridine, anarylquinoline, a pyridylnaphthalene, a pyridylpyrrole, apyridylimidazole and a phenylindole.
 104. The composition of claim 91wherein the X ligand is a substituted or unsubstituted ligand selectedfrom the group consisting of an aminoacid, a salicylaldehyde, anacetylacetonate and a substituted or unsubstituted compound having thestructure:


105. The composition of claim 91 wherein the L ligand comprises asubstituted or unsubstituted arylquinoline.
 106. The composition ofclaim 105 wherein the X ligand comprises an acetylacetonate.
 107. Thecomposition of claim 105 wherein the L ligand comprises an unsubstitutedarylquinoline having the structure:


108. The composition of claim 107 wherein the X ligand comprises anacetylacetonate.
 109. The composition of claim 105 wherein the L ligandcomprises a substituted arylquinoline having the structure:


110. The composition of claim 109 wherein the X ligand comprises anacetylacetonate.
 111. A composition comprising a compound of formulaLL′L″M, wherein L, L′ and L″are inequivalent monoanionic bidentateligands, M is a metal that forms octahedral complexes, and the L, L′ andL″ ligands are coordinated to M through an sp² hybridized carbon and aheteroatom.
 112. The composition of claim 111 wherein the heteroatom ofthe L, L′ and L″ ligands is nitrogen.
 113. The composition of claim 111wherein M is selected from the group consisting of the transition metalsof the third row of the transition series of the periodic table. 114.The composition of claim 111 wherein M comprises Pt.
 115. Thecomposition of claim 111 wherein M comprises Ir.
 116. A compositioncomprising a compound of formula LL′MX, wherein L, L′ and X areinequivalent monoanionic bidentate ligands, M is a metal that formsoctahedral complexes, and the L and L′ ligands are coordinated to Mthrough an sp² hybridized carbon and a heteroatom.
 117. The compositionof claim 116 wherein the heteroatom of the L and the L′ ligands isnitrogen.
 118. The composition of claim 116 wherein M is selected fromthe group consisting of the transition metals of the third row of thetransition series of the periodic table.
 119. The composition of claim116 wherein M comprises Pt.
 120. The composition of claim 116 wherein Mcomprises Ir.
 121. The composition of claim 116 wherein the X ligand isan O—O ligand.
 122. The composition of claim 116 wherein the X ligand isan N—O ligand.
 123. A composition comprising a compound of formulaLMXX′, wherein L, X and X′ are inequivalent monoanionic bidentateligands, M is a metal that forms octahedral complexes, and the L ligandis coordinated to M through an sp² hybridized carbon and a heteroatom.124. The composition of claim 123 wherein the heteroatom of the L ligandis nitrogen.
 125. The composition of claim 123 wherein M is selectedfrom the group consisting of the transition metals of the third row ofthe transition series of the periodic table.
 126. The composition ofclaim 123 wherein M comprises Pt.
 127. The composition of claim 123wherein M comprises Ir.
 128. The composition of claim 123 wherein the Xligand is an O—O ligand.
 129. The composition of claim 123 wherein theX′ ligand is an N—O ligand.